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.

Semantically, if he wanted to create a group discussion, he would have put professor Y and Z in the To field, and would have addressed them and you in the first few lines of the email e.g. "Dear Y, Z and dwoz,..."

If he wanted to let Y, Z know about his correspondence with you, i.e. keep them informed, he would, as he did, put them in the CC field, (For the interested reader later CC=Carbon Copy). He is just ensuring they have knowledge of what went down.

If he wanted to ensure they had knowledge, but you did not know they knew, he would have put them in the BCC field (Blind Carbon Copy).

However, people do not always use the fields "correctly".