Question

which is a complete sentence?
in the midst of ski season, a gigantic avalanche.
elise determined the diameter of the circle.
never spoke of ariels secret to anyone except for her.

Explanation:

A complete sentence must have a subject and a predicate. "In the midst of ski season, a gigantic avalanche" lacks a predicate. "Never spoke of Ariel's secret to anyone except for her" lacks a subject. "Elise determined the diameter of the circle" has a subject (Elise) and a predicate (determined the diameter of the circle).

Answer:

Elise determined the diameter of the circle.