In order to discover what unit or units have activated a trigger, use the list command. E.g.:
this MyTrigger
This will provide an array of all units that currently meet the trigger's conditions. Thus, if you want to save this array, use the following code:
units = list MyTrigger
Or, if this is in a script, use a local variable instead. E.g.:
_units = list MyTrigger
If you just want the first unit that meets the condition, add the select command. E.g.:
(this MyTrigger) select 0
Good luck!