Answer
$\text{A.}$ $3x=24$
Work Step by Step
The phrase "The product of $3$ and a number" means that we multiply $3$ by a variable, x, giving us $3x$. The phrase "is 24" means "is equal to 24," which we express with $=24$. When we combine these two parts, we get $3x=24$, which is choice $\text{A.}$
