Chapter 3 - Equations and Problem Solving - 3.2 - Equations and Problem Solving - Concept Quiz 3.2 - Page 106: 8

$\text{D.}$ $x-3=24$

The phrase "$3$ less than a number" means that we subtract a variable, $x$, from the number $3$, giving us $x-3$. The phrase "is 24" means "is equal to 24", which we express with $=24$. When we combine these two parts, we get $x-3=24$, which is choice $\text{D.}$