Which of the following terms refers to a line of verse containing three metrical feet?

The term that refers to a line of verse containing three metrical feet is indeed trimeter. In poetry, a foot is a basic unit of meter consisting of a combination of stressed and unstressed syllables. Trimeter specifically denotes a line that has three of these feet.

Dimeter, contrastingly, involves two feet per line, and monometer consists of just one foot, making them both shorter than trimeter. Quatrain refers not to the feet in a line but to a stanza or poem that has four lines, which further distinguishes it from the concept of metrical feet in lines of verse. Thus, trimeter accurately describes the structure specified in the question, highlighting its significance in poetic forms.