Class C airspace entry requires two-way radio communication - not a clearance - and contact is established the moment ATC uses your N-number in their response.
PatternFAR 91.129 requires two-way radio communication before entering Class D airspace - that means hearing your call sign in the controller's reply, not just any response.
PatternIn Class Charlie airspace, you're cleared to enter the moment the controller says your callsign back - not when you hear the word 'cleared.'
PatternTo enter Class Charlie airspace you only need two-way radio communication established, which happens the moment ATC says your call sign.
Pattern