To find out how many pounds (x) of apples Bill can buy, we need to set up an equation reflecting the cost of the apples.
The cost of the apples is $1.25 per pound, and he has $5 to spend. Therefore, the equation relating the cost to the amount of money he has is:
\[ 1.25x = 5 \]
where \( x \) is the number of pounds of apples he can buy.
So, the correct answer is: D. 1.25x = 5.