Answer:
C. 5
Step-by-step explanation:
"Polynomial" means consisting of several terms, or more than one term.
In A, we see that there are 2 terms (3x and 5), and so it is a polynomial, specifically a binomial.
In B, we also see 2 terms (2/5x and -21), and so it is also a polynomial, specifically a binomial.
In C, we only see one term (5), and so it is a monomial, or having only one term.
In D, we can see 2 terms (x^-3 and 5x), and so it is also a polynomial, specifically a binomial.
~
