Professor cc to other professors when I ask about the PhD student vacancy. Should I answer as a group or to individuals?
The key factor here is that the professor X is retiring.  He is not attempting to initiate a group discussion, he is trying to pass you off to the other CC professors and give you an introduction to them.

The correct way to respond is to send a nice single-addressee thank you note to original professor X only, perhaps also asking your query about Professor T, then send a single-addressee email to each individual Professor listed in the CC.

The key here is that you won't be working with the group, you'll only be working with one of them.