Antonym of CONVEX

Answer: Option D

Convex : having an outline or surface curved like the exterior of a circle or sphere.
Flat : having a level surface; without raised areas or indentations.
Protuberant : protruding; bulging.
Full : containing or holding as much or as many as possible; having no empty space.
Indented : divided or edged with a zigzag line.

Antonym of Convex is Indented