It’s a typing error - seriously
The correct use of the verb ‘to watch’ in this sentence and context is ‘had been watching’.
Ali came into the sitting room. It was empty, but the television was still on. Someone had been watching it.
The phrase ‘had been watching’ is known as past perfect continuous tense, describing a past action that started and continued up to another action or time in the past.
In this situation it is particularly used to describe an event which finished just before another past event, to express the cause of the second event - in this case, the empty room with the tv ‘still on’.
I have to assume this is a typing error, and that the correct answer should have been 1 (if it had been typed correctly), only because the other three options all show technically correct sentence structure and use of verb when read out of context.
2) Someone watched it.
3) Someone was watching it.
4) Someone had watched it.
Whereas 1) does not create a correctly structured sentence in any situation.
1) Someone had been watched it.
Incidentally, while 4) would appear to be the most correct out of the choices available, it gives no indication that the tv was watched just before Ali entered the room (thereby explaining why it was ‘still on’), only that it was watched at some unspecified time in the past - maybe once last week?