Answer:
D.
Explanation:
A term in a polynomial is a single number or a number multiplied by any number of variables. A polynomial is a sum of terms.
The only thing that can be done with variables in a polynomial is to multiply them by numbers or other variables. In a polynomial, you cannot: divide by a variable (have a variable in a denominator), have a root of a variable, have a log of a variable, have a variable as an exponent, etc.
All expressions are polynomials except for D. In D., the negative exponents mean division by variables.