7x (option C)
Explanation:A monomial consists of one term. It could be a constant, variable or a combination of both.
From the options:
[tex]\begin{gathered} a)\text{ }8x^2+7x+3 \\ \text{This has 3 terms: }8x^2,\text{ }7x,\text{ 3} \\ It\text{ is not a monomial} \end{gathered}[/tex][tex]\begin{gathered} b)\text{ }\sqrt[]{x}\text{ - 1} \\ \text{This has two terms: }\sqrt[]{x}\text{ and 1} \\ It\text{ is not a monomial} \end{gathered}[/tex][tex]\begin{gathered} c)\text{ 7x} \\ \text{This has one term: 7x} \\ It\text{ is a monomial (option C)} \end{gathered}[/tex][tex]\begin{gathered} d)\text{ }\frac{9}{x} \\ A\text{ monomial cannot have a variable in the exponent} \\ it\text{ is not a monomial} \end{gathered}[/tex]