Which of the following is a monomial?

• A. x
• B. x+y
• C. x^2+y^2
• D. 2x+3

Answer: (A)

A monomial is a single algebraic term: a constant times a variable raised to a nonnegative integer power, with nothing added or subtracted. The expression x is just a single term, equivalent to 1·x, so it fits as a monomial. The other expressions each combine two or more terms (x+y, x^2+y^2, and 2x+3), so they are not monomials. Therefore, the monomial here is x.