The reason

Hi everybody,
Which one is correct n looks natural:

A)
1- The pepper! The reason her pizza is delicious.

2- The pepper! This is the reason her pizza is delicious.

3- The pepper! This is the reason why her pizza is delicious.

4- The pepper! This is the reason to why her pizza is delicious.

B)
1- He missed a call from his girlfriend and so he is grouchy now!

2- He missed a call from his girlfriend and so he feels grouch now!

3- He is grouchy because he missed a call from his girlfriend.

C)
1- We should get to work, or we won't be finished by dinnertime.

2- We should get to work, or we couldn't be finished by dinnertime.

3- We should get to work, or we won't be able to finish it by dinnertime.

4- We should get to work, or we couldn't be able to finish it by dinnertime.

Thanks!