Answer:
The correct answer is C) j'ai travaillé. The correct sentence is
L’année dernière j'ai travaillé les lundis, mercredis, et parfois les vendredis.
Explanation:
This is a common confusion but j'ai travaillé means I worked and je travaillais means I was working. The other options (A or B) are not possible because they are not in the past tense and the sentence is in the past because it says last year.