Choice Dhoice C is the correct answer. This straightforward addition equation requires subtraction to solve.

1. Start with the equation: $x + 16 = 40$
2. Subtract 16 from both sides: $x = 40 - 16$
3. Calculate: $x = 24$
4. Verify: $24 + 16 = 40$ ✓

The solving process is consistent: to undo addition, use subtraction.

Choice Dhoice C is incorrect because this results from adding instead of subtracting: $40 + 16 = 56$. Remember, we need the inverse operation.

Choice Dhoice B is incorrect because this confuses the constant term (16) with the solution. The number being added to $x$ is not the value of $x$ itself.

Choice Dhoice D is incorrect because this results from dividing: $\frac{40}{16} = 2.5$. Division is not the appropriate operation for this equation.