Past progressive or past continuous is a tense used to express <u>actions in progress in the past</u>. It can be identified because a) it used the auxiliary be in past (was or were) and b) it requires the verb in its participle form, plus ing, such as write=writing.
Option C describes an action in progress (<em>Melanie was riding</em>) at a specific time in the past (<em>all afternoon</em>) so it is the correct answer.
