After 20 % of the original number of marbles in a group were removed from the

Choice B is correct. It's given that $20\%$ of the original number of marbles were removed from the group. Let $x$ represent the original number of marbles in the group. It follows that $\frac{20}{100}x$, or $0.20x$, marbles were removed from the group. Therefore, $x-0.20x$ marbles remained in the group. It's also given that $360$ marbles remained in the group. Thus, $x-0.20x=360$, or $0.80x=360$. Dividing both sides of this equation by $0.80$ yields $x=450$. Substituting $450$ for $x$ in the expression $0.20x$ yields $0.20\left(450\right)$, or $90$. Therefore, $90$ marbles were removed from the group.

Choice A is incorrect. This is $20\%$ of the remaining number of marbles.

Choice C is incorrect. This is the original number of marbles, not the number of marbles that were removed.

Choice D is incorrect and may result from conceptual or calculation errors.