Choice Choice C is the correct answer. We add to undo the subtraction.

1. Start with the equation: $x - 40 = 25$
2. Add 40 to both sides: $x = 25 + 40$
3. Calculate: $x = 65$
4. Verify: $65 - 40 = 25$ ✓

The pattern holds regardless of the size of the numbers: addition is always the inverse of subtraction.

Choice Choice A is incorrect because this results from subtracting instead of adding: $25 - 40 = -15$. This applies subtraction twice.

Choice Choice B is incorrect because this is the absolute value of Choice Choice A, perhaps from dropping the negative sign: $|{-15}| = 15$.

Choice Choice D is incorrect because this results from dividing: $\frac{25}{40} = 0.625$. Division is not the inverse of subtraction.