Sure, let's break down each question and find the correct answers.

1. **It's cool in the...**
- a) spring
- b) summer
- c) fall
- d) winter

**Answer:** c) fall
- Explanation: Generally, fall (or autumn) is known for its cooler temperatures compared to summer, but it is not as cold as winter.

2. **Hoa and Lan ....their grandfather and grandmother tomorrow**
- a) sees
- b) is going to see
- c) see
- d) are going to see

**Answer:** d) are going to see
- Explanation: "Hoa and Lan" is a plural subject, so the correct form is "are going to see" to indicate a future plan.

3. **We are going to visit our grandparents...one week**
- a) st
- b) on
- c) for
- d) in

**Answer:** c) for
- Explanation: "For" is used to indicate the duration of time. So, "for one week" is the correct phrase to indicate the length of the visit.

So, the correct answers are:
1. c) fall
2. d) are going to see
3. c) for