He has suspended his secretary on a flimsy ground.

Answer: Option D

Flimsy : (of a pretext or account) weak and unconvincing. Vigorous : strong, healthy, and full of energy.
Strong : able to withstand force, pressure, or wear.
Sound : in good condition; not damaged, injured, or diseased.
Salutary : (especially with reference to something unwelcome or unpleasant) producing good effects; beneficial.

Antonym of Flimsy is Salutary