“Enable only the options” from a previous categorical question is done using Filter in Survey Solutions which is described here.

Assuming e1 is Categorical: Multi-Select with Display Mode - Checkboxes add the following syntax to the Filter Box of e2:

> e1.Contains(@optioncode)

If e1 is Categorical: Multi-Select with Display Mode - Yes/No buttions:

> e1.Yes.Contains(@optioncode)

Asking e2 only makes sense if there are more than one sources of water, therefore consider adding an enabling condition on e2 such as: e1.Length>1 or e1.Yes.Length>1