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I figured the answer would be option (B) as the group attached to the benzene ring is ortho- and para- directing.
I thought $\ce{FeBr3}$ would be used in electrophilic aromatic substitution.
However, the model answer to the question is (D).
Is this correct? If so, what makes it possible?
This is a classic $\alpha$-bromination of a carbonyl. You can achieve this with an electrophilic source of bromine like in the reaction conditions provided or something like N-bromosuccinimide.
The reason you're not getting the EAS product is because that's a much higher energy pathway since you need to temporarily break aromaticity. Given more time, heat, and bromine, you can probably get the ortho- and para- products, but you will still $\alpha$-brominate on the amide first.
There are two competing mechanisms.
The first one is the electrophilic aromatic substitution which you expected:
The rate-determining step is presumably the first step, where a relatively unstable arenium ion is formed.
The second one is the carbonyl alpha-substitution reaction:
I failed to identify the rate determining step because of the coordination of the Lewis acid $\ce{FeBr3}$.
Therefore, the second mechanism occurs at a higher rate compared to the first mechanism, so more 2-bromo-N-phenylacetamide is produced per unit time, which makes it the major product.
