Chapter 3 - Equations and Problem Solving - 3.2 - Equations and Problem Solving - Concept Quiz 3.2 - Page 106: 6

$\text{C.}$ $x+3=24$

The phrase "3 added to a number" means that we add $3$ and a variable, $x$. The phrase "is 24" means "equals to," which we express with $=24$. When we combine these two parts, we get $x+3=24$, which is choice $\text{C.}$