Answer:
C. 5
Step-by-step explanation:
"<em>Polynomial</em>" means <em>consisting of several terms</em>, 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 <em>binomial.</em>
In B, we also see 2 terms (2/5x and -21), and so it is also a polynomial, specifically a <em>binomial.</em>
In C, we only see one term (5), and so it is a <em>monomial</em>, 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 <em>binomial</em>.
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