He stood gazing at the serene expanse of the sea.

The correct answer is C. tranquil.

The word “tranquil” means calm, peaceful, and quiet. It is the best word to describe the sea in the sentence “He stood gazing at the serene expanse of the sea.” The sea

is described as being vast and peaceful, and the word “tranquil” captures this feeling perfectly.

The other options are not as accurate. The word “scenic” means beautiful or picturesque. While the sea may be scenic, it is not the most important quality that is being described in the sentence. The word “clear” means transparent or easy to see through. While the sea may be clear, it is not the most important quality

that is being described in the sentence. The word “ruffled” means disturbed or agitated. While the sea may be ruffled, it is not the most important quality that is being described in the sentence.

In conclusion, the correct answer is C. tranquil.