The adjective "lightest" is in the superlative form. Option C is correct.
A superlative adjective shows the extreme or highest degree of a quality. Superlative adjectives are used in order to describe the extreme quality of something in a group of things. Superlative adjectives are used when we are talking about three or more things.
In this particular case, lightest is the superlative form of the adjective light.
