He stood gazing at the serene expanse of the sea.

Answer: Option D

Serene : calm, peaceful, and untroubled; tranquil.
Scenic : providing or relating to views of impressive or beautiful natural scenery.
Clear : easy to perceive, understand, or interpret.
Tranquil : free from disturbance; calm.
Ruffled : disturb the smoothness or tranquillity of.

Antonym of Serene is Ruffled