For the official practise test 2, section 3, question 4 is listed as a “plugging in” question.
So when I tried it out, I substituted 1 for both variables a and b. That easily allows me to pick option A.
However, one of your rules was to never plug in the values 0 and 1, yet 1 was a viable option for this question.
So how can I know when plugging in 1 will be okay and when it’s a bad idea? Also, is my method correct for this question or would you suggest another way? Thanks so much!

The reason you generally want to avoid plugging in 1 or zero is that doing so may result in more than one answer being correct. This is also the reason you generally should not plug the same number in multiple times in the same question. As you point out, though, you got away with doing both things here: setting both a and b equal to 1 in this case worked very nicely, eliminating all choices but the right one.

Recognize that plugging in other numbers would also work here—that’s the beauty of plugging in. In fact, you could even have plugged in the other number to avoid, zero, for one variable and gotten away with it! (Plugging in zero for both variables wouldn’t eliminate any choices, though.)

As for how you can know when plugging in 1 will be OK, if you want to plug in 1 just make sure you check every answer choice (which you should be doing anyway when you plug in). If plugging in 1 left more than one choice, plug in a new number and try the choices you didn’t eliminate again.