#### Answer

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

#### Work Step by Step

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