ANS:C - It does not respond to many solutes

The answer (C) is wrong. The correct one is B.

Refractive index is a physical constant which many compounds have NEARLY the same value.(similar to melting point) = not specific, respond to many compounds. C - WRONG.

If the mobile phase come along with very small amount of the analyte, no change in the signal (refractive index of the mobile phase is still the same) = not sensitive. A - WRONG.

But if the composition of mobile phase is changed (gradient), large amount (compare to the analyte) of quantity in the composition is change, then the signal (refractive index) will change due to the change in composition of mobile phase. This huge change will overlay the signal due to the analyte which is much less than mobile phase. B - CORRECT.