Tense puzzle

Hi!

Below is a set of 4 sentences. Each sentence is accompanied by a few answers. Could you tell me why a particular answer is correct or incorrect? The answers marked with "(OK)" sound good so I do not need any explanation but I'm thinking about the other options. Could you help me?

1.

A: Have your parents decided whether you can come to the festival next weekend?

B: Not yet, but they can't stop me I ..... (come) with you

a) will come (OK)

b) I'm going to come (OK)

c) I'm coming

2.

I've won the jackpot on the lottery and I .... (spend) it all straight away!

a) am going to (OK)

b) will spend

3.

A: Have you looked at the new financial report?

B: No, but I.... (stay) at home this evening so I can study it then

a) am staying (OK)

b) are going to stay

c) will stay

d) wil be staying

4.

Erm, I don't want to be rude, but will .... (stay) with us for long when you come over to Britain?

a) will you be staying (OK)

b) are you going to stay (OK)

c) are you staying