The correct option is C opaque
The word 'transparent' refers to something that allows light to pass through so that the objects can be distinctly seen.
Option A, opaque refers to something not transparent.
Option B, translucent refers to something not transparent.
Option C, solid refers to something firm and stable in shape.
Option D, fragile refers to something that can be easily broken or damage.
Thus, according to the meaning, the word 'opaque' is the opposite of the word 'transparent'.