Compound (X) [styrene] is subjected to the sequence of reactions as...

Addition of $Br_2$ to styrene (X) yields 1,2-dibromoethylbenzene.

$C_6H_5-CH=CH_2 \xrightarrow{Br_2/CHCl_3} C_6H_5-CH(Br)-CH_2(Br)$

Reaction with excess $NaNH_2$ causes double dehydrohalogenation followed by deprotonation of the terminal alkyne, forming sodium phenylacetylide.

$C_6H_5-CH(Br)-CH_2(Br) \xrightarrow{NaNH_2 \text{ (excess)}} C_6H_5-C \equiv C^- Na^+$

Nucleophilic substitution with $CH_3I$ yields 1-phenylpropyne.

$C_6H_5-C \equiv C^- Na^+ \xrightarrow{CH_3I} C_6H_5-C \equiv C-CH_3$

Reduction with $Na/NH_3(l)$ reduces the internal alkyne to a trans-alkene without affecting the benzene ring.

$C_6H_5-C \equiv C-CH_3 \xrightarrow{Na/NH_3(l)} C_6H_5-CH=CH-CH_3$

The major product (Y) is trans-1-phenylpropene, which has the molecular formula $C_9H_{10}$.

Molar mass of $C_9H_{10} = (9 \times 12) + (10 \times 1) = 108 + 10 = 118 \text{ g mol}^{-1}$.

Answer: $118$