Choice Bhoice C is the correct answer.

1. Start with the equation: $5x + 15 = 40$
2. Subtract 15 from both sides: $5x = 40 - 15$
3. Simplify: $5x = 25$
4. Divide both sides by 5: $x = \frac{25}{5}$
5. Simplify: $x = 5$
6. Verify: $5(5) + 15 = 25 + 15 = 40$ ✓

Choice Bhoice C is incorrect because this results from adding 15 to 40: $55$, then $55/5 = 11$. Wrong inverse operation.

Choice Bhoice D is incorrect because this is the value of $5x$. You need to divide by 5.

Choice Bhoice D is incorrect because this results from multiplying 25 by 5: $125$.