The correct structure of C is

The reaction sequence begins with propanenitrile (CH₃-CH₂-CN) reacting with methylmagnesium bromide (CH₃MgBr), a Grignard reagent, in the presence of dry ether. The nucleophilic methyl group from the Grignard reagent attacks the electrophilic carbon of the nitrile group, forming an intermediate imine salt designated as compound A (CH₃-CH₂-C(CH₃)=NMgBr).

This intermediate is then immediately subjected to acidic hydrolysis (H₃O⁺), which cleaves the carbon-nitrogen double bond to yield a ketone. Consequently, compound B is butan-2-one (CH₃-CH₂-CO-CH₃).

In the final step, butan-2-one undergoes a Clemmensen reduction using zinc amalgam (Zn-Hg) and concentrated hydrochloric acid (HCl). This reduction explicitly targets the carbonyl group, completely deoxygenating it and converting the >C=O group into a methylene (-CH₂-) group. Therefore, the final product, the correct structure of C, is the straight-chain alkane butane (CH₃-CH₂-CH₂-CH₃).