Sure, here are the correct answers for each of the questions:

1. "A kilo of rice, what ... would you like?"
- c. else
- Correct sentence: "A kilo of rice, what else would you like?"

2. "My sister needs ... of beef."
- c. half a kilo
- Correct sentence: "My sister needs half a kilo of beef."

3. "Lan and Hoa are going ... vacation this summer."
- a. on
- Correct sentence: "Lan and Hoa are going on vacation this summer."

So the correct answers are:
1. c. else
2. c. half a kilo
3. a. on