The output from first two NOR gates would be $\overline{A},\overline{B}$ respectively(complementing action performed by NOR gates)
From the third NOR gate the output would be, $\overline{\overline{A}+\overline{B}}$ $=$ $\overline{\overline{AB}}=AB$ (using De-morgan's law)
So, output from the fourth NOR gate would be: $\overline{AB+AB}$ $=$ $\overline{AB}$
Option B is correct.