Final answer:
Gregor's exoskeleton is predominantly described as dull brown in color.
The correct option is A.
Step-by-step explanation:
From the description provided by Grete regarding Gregor's condition, the predominant color of Gregor's exoskeleton is dull brown.
This is explicitly stated in the text, noting that his exoskeleton was a dull brown color.
Gregor's new appearance is a significant contrast to his former human physique, and his numerous limbs and dark eyes further emphasize the stark transformation he experiences into a grotesque insect-like being.
The correct option is A.