The degree of a polynomial is the value of the highest exponent a variable is raised to (i.e., the degree of the equation 
is 2, since the highest exponent of a variable is 2.
If we apply this concept to your list of polynomials, we see that polynomial A and polynomial D both have
terms, so they are both fifth-degree polynomials. However, your answer must also be a trinomial (a polynomial with three terms.) If we look at polynomials A and D, we see that only polynomial A has three terms, so that must be the answer!
Let me know if you have any questions :)
If we apply this concept to your list of polynomials, we see that polynomial A and polynomial D both have
Let me know if you have any questions :)