Is D flip flop can be combination circuit?
You said, D flip flop have characteristic equation Q(t+1)= D which means here also output at any time follow the input only"

But that's not true -- the output of the D flip-flop changes state only when the CLK input goes high (rising edge trigger).  At that time, Q = D.  All other times, D can change and it has no effect on Q.

So the D flip-flop is sequential (has state), not combinatorial.