"A machine is having issues, so the user is responsible for getting the machine serviced. Which stage of the hardware lifecycle does this scenario belong to?"

Maintenance

Explanation:

The maintenance stage of an hardware life-cycle involves the servicing, repair replacement of machine component activities as well as other restorative activities meant to restore the machine to its original state.

The stages of an hardware life-cycle are;

1) Planning for the required hardware

2) Buying or leasing the hardware

3) Hardware installation

4) Maintenance activities, Repair

5) Hardware upgrading

6) Extend

7) Hardware trade in or buyback

8) Disposal of the hardware

I think its the second one