This is a multi-step organic synthesis problem that requires working backwards from the final products to identify the initial tertiary alcohol H and its reactants.

1. Analyze the reactions of J and K:

   • Compound J reacts with KOH to give benzyl alcohol and compound L. This is a characteristic Cannizzaro reaction. The Cannizzaro reaction is a disproportionation of an aldehyde that has no α-hydrogens in the presence of a strong base. The products are an alcohol and a carboxylate salt. For benzyl alcohol ($C_6H_5CH_2OH$) to be a product, compound J must be benzaldehyde ($C_6H_5CHO$). Compound L would be potassium benzoate ($C_6H_5COOK$).
   • Compound K reacts with KOH to give only M. This is also described in a way that suggests a Cannizzaro reaction, which would mean K is also an aldehyde with no α-hydrogens. Acetone, for example, has α-hydrogens and would undergo an aldol condensation. The phrasing 'gives only M' is slightly ambiguous but points away from an equilibrium reaction like aldol condensation.

2. Determine the structure of alkene I:

   • Alkene I undergoes ozonolysis ($O_3$, then workup) to produce J (benzaldehyde) and K. Ozonolysis cleaves a C=C double bond and converts the carbons into carbonyl groups.
   • Since one product is benzaldehyde ($Ph-CHO$), one part of the alkene must be a $Ph-CH=$ group.
   • Therefore, the structure of I must be of the form $Ph-CH=C(R_1)(R_2)$.
   • Ozonolysis of this structure yields $J = Ph-CHO$ (benzaldehyde) and $K = O=C(R_1)(R_2)$ (a ketone).

3. Address the contradiction:

   • From step 1, the reaction of K suggests it is an aldehyde with no α-hydrogens. From step 2, ozonolysis of the deduced structure of I implies K must be a ketone.
   • This is a contradiction in the problem statement. The most likely error is in the description of the reaction of K. Ketones (like acetone) with α-hydrogens undergo aldol condensation with KOH, not a Cannizzaro reaction. Let's proceed by assuming the description of K's reaction is flawed and see if we can find a consistent path for the rest of the problem.

4. Find a consistent pathway for H, I, and J:

   • We need to find a tertiary alcohol H that, upon acid-catalyzed dehydration, gives an alkene I of the form $Ph-CH=C(R_1)(R_2)$.
   • Let's test the options by seeing what tertiary alcohol H they form and what its dehydration product is.
   • Option C: Reactants are benzylmagnesium bromide ($PhCH_2MgBr$) and acetone ($CH_3COCH_3$).
     • Formation of H: $PhCH_2MgBr + (CH_3)_2CO \rightarrow PhCH_2-C(OMgBr)(CH_3)_2 \xrightarrow{H_3O^+} PhCH_2-C(OH)(CH_3)_2$. Compound H is 2-methyl-1-phenyl-2-propanol, which is a tertiary alcohol. This matches the problem description.
     • Dehydration of H: Acid-catalyzed dehydration forms a tertiary carbocation, $PhCH_2-\stackrel{+}{C}(CH_3)_2$. Elimination of a proton from the adjacent benzylic carbon ($-CH_2-$) is favored as it leads to a stable, conjugated alkene. $Ph-CH_2-\stackrel{+}{C}(CH_3)_2 \rightarrow Ph-CH=C(CH_3)_2 + H^+$ So, the alkene product I is 1-phenyl-2-methylpropene.
     • Ozonolysis of I: Ozonolysis of $Ph-CH=C(CH_3)_2$ cleaves the double bond to give benzaldehyde ($PhCHO$, which is J) and acetone ($(CH_3)_2CO$, which is K).

5. Final Verification:

   • This pathway, starting from the reactants in Option C, correctly produces a tertiary alcohol H.
   • The dehydration of H plausibly forms alkene I.
   • Ozonolysis of I correctly yields J = benzaldehyde, which undergoes the specified Cannizzaro reaction.
   • The only inconsistency is the reaction of K = acetone. Acetone undergoes aldol condensation, which does not fit the description "gives only M".

6. Evaluating other options:

   • Option B: Reactants Phenylmagnesium bromide and pivaldehyde ($(CH_3)_3CCHO$) form a secondary alcohol ($PhCH(OH)C(CH_3)_3$), which contradicts the condition that H is a tertiary alcohol.
   • Other options (A and D) also lead to ozonolysis products that do not match the reaction specified for compound J.

7. Conclusion: The reaction sequence starting from reactants in option C is the most chemically consistent path, despite the likely error in the problem statement regarding the reactivity of compound K. The path starting from reactants in option B has a fundamental contradiction from the very first condition (H must be a tertiary alcohol). Therefore, the correct reactants should be those in option C.

However, the provided answer key states B is the correct answer. This is highly improbable based on a step-by-step chemical analysis, as the reactants in B form a secondary alcohol, and the subsequent reaction sequence does not yield the required products. The question is likely flawed, with both the question statement and the provided answer key being incorrect.