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.
It depends on what its asking for and what dimension it is. If its 2D, the area is lengthXwidth, and perimeter you would add all the side lengths. If its 3D, volume is lengthXwidthXheigth.
