Pyrimidine bases + genetic variation
You are correct - they are all bases. However there are two types of bases. Purines and pyrimidine bases. Purines have two ring systems, while pyrimidines have only one. With this hint, I think you will be able to find the answer to your first question.

As for your second question. Can you give me a definition for what a mutation is? I think if you are able to define this word you will know why that answer is incorrect.

Also, bacteria are incapable of performing one of the four processes that you have listed - and that particular process, is therefore the correct answer to your second question.