This is an assignment. We will guide you but not simply answer the task for you.
You know the function of the little triangle with the dot at the bottom of the circuit?
P and Q are the inputs. Assume 0 V = logic low, high voltage = logic high (for the purpose of this task it is not relevant how high teh high voltage is. If it helps you understanding teh circuit, assume logic high = 5V).
Make a
truth table with P, Q as inputs (columns) and Y as output (cells). For each combination of P and Q consider the output Y of the circuit (for simplicity, assume PMOS = on for Vgate = low, NMOS = on for Vgate = high and vice versa).
Check your result against the truth tables for XNOR, XOR, NOR and NOR. The one that matches your result is the answer.
If you like, show us your result (truth table and answer) to cross-check it.