I had problems on the same question too!
When p=(-g) means if g= 3 then p is the opposite and is p=-3.
Let's examine the answer choices
A is correct because G=3 and P=-3
B, C, D are incorrect because they don't have the opposites.
Look at the photo I took of your book and showed how it's correct.
Hope This Helped! Any questions feel free to message Me.