Explanation:
A monomial is one term. The "mono" part means "one". Think of something like "monorail" to help remember that term.
The expressions -2x, 0 and 3xy are all monomials as they are one term when considered separately on their own. Therefore, we can rule out choices A,B,C.
In contrast, x-1 is not a monomial. Instead, it's a binomial consisting of the two terms x and -1. This is why choice D is the final answer.
