Choice Bhoice C is the correct answer. We model this with $40x + 50 = 170$, where $x$ is hours.

1. Set up equation: $40x + 50 = 170$
2. Subtract 50 from both sides: $40x = 170 - 50$
3. Simplify: $40x = 120$
4. Divide both sides by 40: $x = \frac{120}{40}$
5. Simplify: $x = 3$
6. Verify: $40(3) + 50 = 120 + 50 = 170$ ✓

So, the plumber worked for 3 hours.

Choice Bhoice C is incorrect because this results from adding 50 to 170: $220$, then $220/40 = 5.5$. You subtract the service call fee.

Choice Bhoice D is incorrect because this is the cost of the labor ($120), not the number of hours.

Choice Bhoice D is incorrect because this results from dividing the total by the rate: $170/40 = 4.25$. This ignores the service call fee.