The correct answer is b.
A polynomial is an expression that contains terms with the form ax^n where a is equal to any number (negative or positive) and n is the power, or exponent, of x (squared, cubed, etc.). In order for it to be a polynomial, the exponent must be positive. Terms that contain square roots are not polynomials as the exponent is considered radical (1/2). Thus, b is the correct answer.