In order to find this out,, we need to know that $12 is the changing number. This number depends on how many shirts were bought,, so we must put this with x. We also know that $5 is our constant (does not change) and we spent a total of $77 so our equation should equal to 77. This means that our equation should look like the following:
12x + 5 = 77
This means that the correct answer to your question is going to be option C.
Let me know if you have any further questions.
:)