Answer:
1) Let's
2) thanks
3) free
4) sounds
5) How
6) great
Explanation:
Basically, the choice of the adequate word depends on the context of the sentence.
Sentence 1) expresses a suggestion which means that "Let's" seems like a good choice.
Sentence 2) is refusal and polite way to refuse an offer is by using "No, thanks".
Sentence 3) checks whether someone is available, so we can use the adjective "free".
In sentence 4) the proposal is agreed upon, so we use the phrase "That sounds great".
Sentence 5) is another proposal with the phrase "How about...?"
And finally, sentence 6) expresses confirmation with "That's perfect!"
