They have been cleaning.
How can I change it into the passive voice? I'm supposed to change it into the passive voice and I don't know how.
They have been cleaning.
How can I change it into the passive voice? I'm supposed to change it into the passive voice and I don't know how.
This would need "do support" or for an object to be added in order to be converted into a passive construction.
The best we could do without further information would be something that may look quite strange to many eyes, but such utterances are grammatical and natural in certain situations:
[The] cleaning has been being done by them.
More frequently, probably, with a dummy object:
It has been being cleaned by them.
What we can really determine is that whoever chose this for an item on a paper doesn't know English very well.
Related (has been being:)