What is the missing starting material for the following reaction sequence?

Question

Quizplus · Accepted Answer

The reaction sequence shows a Grignard reaction followed by a reaction with a carbonyl compound. Grignard reagents are usually prepared by reacting an alkyl or aryl halide with magnesium metal in dry ether. The missing starting material in this reaction sequence is likely an alkyl or aryl halide, which can be identified by its unique identification number. Among the options given, only choice B has the correct identification number for an alkyl halide.

What Is the Missing Starting Material for the Following Reaction