A difficult drill from a British English textbook

Hello,

Could you please take a look at the following assignment (marked red, in italics, borrowed from a BrEng textbook, intermediate)

Choose the sentence that is most appropriate. In one question, both are possible.

1) (spoken)

1. They’re giving me a promotion at work
2. A promotion is being given to me at work

2) (TV news item)

1. A two-percent rise in temperatures is predicted next year
2. They predict there will be a two-percent rise in temperatures next year

3) (celebrity magazine)

1. The academy nominated the actress Mirta Street for two film awards last year
2. The actress Mirta Street was nominated for two film awards last year

4) (spoken)

1. Their parents have given them a lot of presents
2. A lot of presents have been given to them by their parents

5) (notice on the wall)

1. You aren’t allowed to smoke here
2. Smoking is not allowed here

6) (spoken)

1. I won a prize in an online competition
2. A prize was won by me in an online competition

To begin with, what does it mean, "choose the sentence that is most appropriate"?

Does it imply that for each pair (but one) one sentence is grammatically correct while the other is not?

Could you please comment on each of the six pairs in this drill?

It really beats me why e.g. a) "You aren't allowed to smoke here" is bad grammar.... and what is wrong with b) "Smoking is not allowed here"???