Answer: D) 5
Reason: It's the largest exponent
When a polynomial is written in standard form like this, the term with the largest exponent is written first, then the next largest and so on. So in cases like this, we simply need to look at the left-most term; however, this may not always be the case as your teacher could easily mix up the terms to make sure you're paying attention.
This is considered a quintic polynomial due to the degree of 5.