To set up the equation for this problem, we start by noting that Bill has $5 to spend on apples that cost $1.25 per pound. If we let \( x \) represent the number of pounds of apples Bill can buy, we can write the equation based on his total spending:
\[ 1.25x = 5 \]
This means the cost of the apples (which is the price per pound multiplied by the number of pounds) is equal to the amount of money he has ($5).
So the correct response from your options is:
1.25x = 5