Here are the corrections for each sentence:

1. **Let's growing a lot of plants in our school.**
- Correction: **Let's grow a lot of plants in our school.** (B)

2. **Does your cousin volunteer to join some community activities?**
- Correction: No mistake.

3. **You should eat much fast food and sugar.**
- Correction: **You should eat less fast food and sugar.** (A)

4. **Last Sunday, Mr. Brown divided his volunteers by four groups.**
- Correction: **Last Sunday, Mr. Brown divided his volunteers into four groups.** (D)

5. **Exercise daily at least an hour is good for our health.**
- Correction: **Exercising daily for at least an hour is good for our health.** (A)