Here are the corrections needed for each sentence:

38. Even though exercising **regular** is essential for a healthy lifestyle, maintaining a balanced diet is equally important.
**Correct Answer: B. regular** (It should be "regularly.")

39. I have eaten a balanced diet last week and it has improved my energy level **a lot** since then. You should try it out.
**Correct Answer: A. have eaten** (It should be "I ate a balanced diet last week.")

40. Despite being aware of the negative effects of fast food, Sarah has frequently enjoyed **them.**
**Correct Answer: D. them** (It should be "it" instead of "them" since "fast food" is singular.)

So, the final answers are:
- 38: B
- 39: A
- 40: D