Reactivity of Grignard reagent
The answer will be (a) Acyl chloride.
The acyl carbon in an acyl chloride is connected to the chlorine atom. While you have acknowledged this fact, you have missed the point that the +M (resonance) effect of Chlorine is overcome by the -I (inductive) effect. Thus, the electrophilicity of the acyl carbon is actually greater than the electrophilicity of the carbonyl carbon in an aldehyde. This jump in electrophilicity is so significant that the difference in steric hindrance is neglected.
In fact, this the major reason why acyl chlorides are considered to be one of the best electrophiles in all of organic chemistry.
If Acyl bromides were used, then they would have an even greater electrophilicity and it would be the answer to this question.
Hope this answers your question!