To solve the problem, we need to define the original price of each apple, represented by \( x \). Given that there is a 25 cent discount per apple, the price after the discount is \( x - 0.25 \).
If 12 apples are purchased for a total of $15, we can set up the equation based on the number of apples and their discounted price:
\[ 12 \text{ apples} \times (x - 0.25) = 15 \]
This leads us to the equation:
\[ 12(x - 0.25) = 15 \]
Therefore, the correct choice is:
D. 12(x - 0.25) = 15.