The answers are A and C.

Options A (capricious) and C (unpredictable) are the most suitable choices for conveying the intended meaning because the sentence emphasizes the challenge of placebo responses being unpredictable, with people showing different reactions to the same inert pill or potion.

A. capricious: Capricious refers to something that is impulsive or unpredictable. In the context of placebo responses, this word fits well as it captures the idea that the reactions to the inert pill or potion can be erratic and unpredictable.
B. illusory: Illusory means deceptive or misleading, which is not the primary characteristic described in the sentence. The focus is on the variability of responses rather than the deceptive nature of the placebo.
C. unpredictable: Unpredictable accurately describes the variability of responses to placebos as highlighted in the sentence. It conveys the idea that the reactions are difficult to foresee or anticipate.
D. chimerical: Chimerical means fanciful or imaginary, but it doesn't precisely convey the idea of variability in reactions, making it less suitable in this context.
E. marginal: Marginal means small or insignificant, which doesn't align with the primary challenge described in the sentence regarding the wide variation in responses to placebos.
F. ephemeral: Ephemeral means short-lived or transient, which is not the main characteristic of placebo responses described in the sentence. The focus is on the variability rather than the brevity of the effects.