Identify the three substrates $P, Q, R$ from the reaction sequence idea

The statement says $X$ is a heteroatom (not $C$ or $H$) and is different in $P, Q, R$. In such standard haloalkane/hetero-substituted alkyl comparison questions, the three compounds are alkyl derivatives with different leaving groups:

• $P : R-\mathrm{F}$
• $Q : R-\mathrm{I}$
• $R : R-\mathrm{Cl}$

or equivalently the comparison among alkyl fluorides, chlorides, iodides. The trends asked (bond length, bond enthalpy, $S_N2$ reactivity, and $pK_a$ of conjugate acids of leaving groups) are exactly the standard leaving-group trends.

From the options, we can infer the order being tested is among:

• strongest bond: $\mathrm{C-F}$
• weakest bond: $\mathrm{C-I}$
• best leaving group: $\mathrm{I^-} > \mathrm{Cl^-} > \mathrm{F^-}$

Bond length trend

Bond length increases with size of halogen: $\mathrm{C-F} < \mathrm{C-Cl} < \mathrm{C-I}$

If we map: $P=\mathrm{R-F},\quad R=\mathrm{R-Cl},\quad Q=\mathrm{R-I}$ then $Q > R > P$

So Option A is correct.

Bond enthalpy trend

Bond enthalpy decreases as bond length increases: $\mathrm{C-F} > \mathrm{C-Cl} > \mathrm{C-I}$

With the same mapping: $P > R > Q$

Option B says: $R > P > Q$ which is incorrect.

So Option B is false.

$S_N2$ reactivity trend

For alkyl halides in $S_N2$, better leaving group means higher reactivity: $\mathrm{RI} > \mathrm{RCl} > \mathrm{RF}$

Thus: $Q > R > P$

Option C says: $P > R > Q$ which is opposite.

So Option C is false.

$pK_a$ of conjugate acids of leaving groups

Leaving groups are: $\mathrm{F^-},\ \mathrm{Cl^-},\ \mathrm{I^-}$ Their conjugate acids are: $\mathrm{HF},\ \mathrm{HCl},\ \mathrm{HI}$

Acid strength: $\mathrm{HI} > \mathrm{HCl} > \mathrm{HF}$

Therefore $pK_a$ order is reverse: $pK_a(\mathrm{HF}) > pK_a(\mathrm{HCl}) > pK_a(\mathrm{HI})$

Hence: $P > R > Q$

Option D says: $R > Q > P$ which is incorrect.

So Option D is false.

Final conclusion

The only correct statement is: $\boxed{A}$

Comparison with stored correct answer

Stored correct answer is B, but from standard trends in alkyl halides/heteroatom leaving groups, B is incorrect and A is correct.