Fill in the blank with an appropriate option.
Lecanemab was recently reported to have a small but ______ effect in reducing cognitive decline.

Correct option is B

The correct answer is B: Significant.

Solution Explanation
The sentence uses the coordinating conjunction "but", which indicates a contrast between two ideas.

The Contrast: The first part of the sentence mentions that the effect of the drug (Lecanemab) was "small". To contrast this positively in a medical or scientific context, the drug's effect must still be meaningful, measurable, or important.

Why "Significant" fits: In medical science, a "small but significant" effect means that while the absolute change might look minor, it is statistically proven to make a real, undeniable difference in patients' lives.

Why the Other Options Are Incorrect
A) Microscopic: This means too small to be seen without a microscope. It doesn't make sense to say an effect is "small but microscopic," as those mean essentially the same thing (no contrast).

C) Optional: This means left to choice. Medical effects or outcomes cannot be "optional."

D) Marginal: This means minor or barely existing. Saying "small but marginal" offers no contrast, as both words imply the effect is unimportant.