How can I find the characteristic table of a D Flip Flop?
The outputs of flipflops are commonly denoted with Q and Q'. This is the case here. As for the truth-table: One way to do it is to have a truth table with not 2 but 3 parameters as entry - your two inputs and the previous state (in other words, Q). You thus see what Q becomes, given a certain previous Q, and the inputs C and X.
Note that with feedback, it's possible to have "undefined" states - where you get contradictions or oscillations (A becomes true and makes B false, which makes A false, which makes B true, which makes A true, Which makes B false, .....)