Identify the carbonyl compound from the formula $C_8H_8O$

A Grignard reagent $C_2H_5MgBr$ adds an ethyl group to a carbonyl compound.

The product after hydrolysis is said to react instantly with Lucas reagent, which means the alcohol formed is a tertiary alcohol.

Therefore, the starting compound $C_8H_8O$ must be a ketone (since Grignard addition to a ketone gives a tertiary alcohol).

A ketone with formula $C_8H_8O$ is acetophenone: $C_6H_5COCH_3$

Reaction with ethylmagnesium bromide

Acetophenone reacts with $C_2H_5MgBr$ as: $C_6H_5COCH_3 + C_2H_5MgBr \rightarrow C_6H_5C(OMgBr)(CH_3)(C_2H_5)$

On hydrolysis: $C_6H_5C(OH)(CH_3)(C_2H_5)$

So compound $A$ is a tertiary alcohol, namely: $\text{2-phenyl-2-butanol}$

Reaction with Lucas reagent

Lucas reagent converts tertiary alcohols immediately to alkyl chlorides: $C_6H_5C(OH)(CH_3)(C_2H_5) \xrightarrow[\text{ZnCl}_2]{\text{conc. HCl}} C_6H_5C(Cl)(CH_3)(C_2H_5)$

Thus compound $B$ is: $C_6H_5C(Cl)(CH_3)(C_2H_5)$

Check molecular formula

Phenyl group: $C_6H_5$

Other groups attached to central carbon: $CH_3$ and $C_2H_5$

Total: $C = 6+1+2+1(central\ carbon)=10$ $H = 5+3+5=13$ and one Cl.

So formula is: $C_{10}H_{13}Cl$

This matches the given formula.

Conclusion

Compound $B$ is: $\boxed{C_6H_5C(Cl)(CH_3)(C_2H_5)}$

This corresponds to option C.