Answer: c. Ten years, because maintenance costs don't increase.
Step-by-step explanation:
With the maintenance costs constant at $24,000 a year, the machine is still expected to go 10 years before it's x-ray source is depleted and it has to be scrapped.
This means that the useful life is therefore 10 years because the maintenance cost will not increase but will still keep the machine going for 10 years.