(A) its effect on Federally Insured financial firms rather than to the drop in the overall - Eliminated. "Rather than to" Unidiomatic usage.
(B) its affect on Federally Insured financial firms, as opposed to the drop in the overall - "opposed to" is not the correct idiom usage here. "results from A rather than from B". Eliminated.
(C) its effect on Federally Insured financial firms, rather than from the drop in the overall - "rather than from.. " is not an IC and hence does not need a comma. Comma splice issue. Eliminated
(D) its effect on Federally Insured financial firms rather than from the drop in the overall - Correct. Comma error in C correct.
(E) the effect on Federally Insured financial firms, rather than to the drop in the overall - Eliminated. Changes meaning. The effect is of FRB's tight money policy. This makes it seem like 2 independent reasons.