Choose the correct alternative out of the four and fill in the blank. Ramu is in the habit of revising just once, but Suresh never understands a subject ______ he revises it for at least three times.

Options

A. a) in case
B. b) thus
C. c) although
D. d) unless

Answer

D

Explanation

unless (Conj.) : except under the circumstances that, used for saying that something can only happen or be true in particular circumstances
Here, unless is the right usage.