Final answer:
The set of monomials is not closed under multiplication because the product of two monomials could be a polynomial.
Step-by-step explanation:
Option B is the correct answer: No; the product of two monomials could be a polynomial.
A monomial is an algebraic expression with one term, such as 5x or -2y^2. When you multiply two monomials, the result can sometimes have more than one term, which makes it a polynomial. For example, if you multiply 3x and 2y, the product is 6xy, which is a polynomial. Therefore, the set of monomials is not closed under multiplication.