Answer:
The expression for this circuit is:
!(!(A&&B) || (B&&C) || (C&&D))
The truthtable is in the picture below.
Step-by-step explanation:
To make the output high, all inputs for the NOR gate must be low. For that, A and B must be high, there is no other way.
That implies that C must be low, to make the second AND gate low.
Finally, input D doesn't play a role in the output.