1. They (walk) in the park yesterday. a) walked b) was walking c) were walking

The correct answer is **a) walked**. **Explanation:** The sentence describes an action that happened at a specific time in the past (yesterday). Therefore, we need to use the simple past tense of the verb "walk," which is "walked." The options "was walking" and "were walking" represent the past continuous tense which is used for actions in progress in the past, not the case here.