Hey there!
A monomial is a polynomial (which is an expression or equation with any amount of terms, though polynomial typically refers to four or more) that contains only one term.
A term can be a number, a variable, a variable with an exponent or a coefficient (or both), or really anything else that you can think of that's separated from another term with some sort of math sign (addition, division, etc.).
Let's take a look at your answer choices:
12c (one term)
c² – 16 (two terms)
c² + c + 6 (three terms)
c³ + 4c² – 12c + 7 (four terms)
The only polynomial that has one term is your first answer choice. The second answer choice is a binomial, the third is a trinomial, and the fourth is just called a polynomial. Like I said, anything that is four terms or more is a polynomial and doesn't have its own name.
Your answer will be 12c.
Hope this helped you out! :-)