The signal A is given to a NOR gate and B is given to an OR gate. The output of these two signals is given to a NAND gate. Using the laws of Boolean algebra,
Y=A⋅Bˉ=A+Bˉ.
Analysing from the input graphs, one can see,
t1−t2:A=0,Bˉ=0,Y=0t2−t3:A=1,Bˉ=0,Y=1t3−t4:A=0,Bˉ=1,Y=1t4−t5:A=1,Bˉ=0,Y=1t5−t6:A=1,Bˉ=1,Y=1