Assortment of Grammatical Sentences

In How to Teach Grammar by Scot Thornburry, p. 71, there are a few sentences that he says are all grammatical. Can you exaplain the ones underlined?

What's more, the decontextualizing of grammar often results in practice exercises that are of doubtful value. For example:
1. Choose the correct form of the verb:
a. Do you work / Are you working every weekend?
b. "Cigarette?" "No thanks, I'm not smoking/ I don't smoke."
c. "What do you eat/ are you earting?" "Cake."
2. Which of these sentecnes are grammatically correct?
a. I'm planning to go to India for my holidays.
b. "The phone's ringing!" " I'm going to get it!"
c. They will have a party next week.
d. I'm tired. I think I'm going to bed.
The point here is that none of these examples has a clear "right answer" and a clear "wrong" one. They are all well-formed sentecnes (that is, they are grammatically accurate), even though we recognise some choices as being more likely than others. But it's possible to imgagine a context where , for example, "Cigarette?" "No, thanks, I'm not smoking" is perfectly appropriate. . . .

I can guess why some of those choices are possible after pondering the other possibilities and why would the other grammatical forms be possible. But can anyone elaborate a bit on 2c?