Final answer:
The expression that is not a polynomial is (b) 4.
Step-by-step explanation:
A polynomial is an algebraic expression that consists of variables and coefficients, combined using addition, subtraction, and multiplication, with non-negative integer exponents. So, in order to determine which expression is NOT a polynomial, we need to identify any expressions that violate this definition.
(b) 4 is not a polynomial because it is a constant term and does not have any variables or exponents. A polynomial must have at least one variable term.
Therefore, the correct answer is (b) 4, which does not satisfy the definition of a polynomial.