The question image/text is incomplete: the reactant structure and the option structures are missing. So the exact product cannot be derived from the provided statement alone.

However, the phrase "first step of the reaction with excess $\mathrm{Mg/Et_2O}$" in the chapter haloalkanes and haloarenes usually refers to formation of a Grignard reagent from every halogen atom that can react with magnesium:

If the substrate contains more than one halogen and magnesium is used in excess, then in the first step all suitable halogens are generally converted into corresponding organomagnesium groups.

But since:

1. the substrate is not visible, and
2. options A, B, C, D are also not shown,

it is impossible to verify the exact structure of the product from the given data.

Comparison with stored answer

The stored correct answer is A, but because the actual option structures are absent, I cannot independently confirm it rigorously.

So the best I can do is note that the provided question is incomplete. If you share the missing reactant structure and option figures, I can solve it fully and verify whether A is correct.