JEE Main · 2021 · 26 Feb 2021 · Shift 2 · Semiconductors

A NAND gate with both inputs tied to the same line gives $\overline{A\cdot A}=\bar{A}$. That output is combined with $B$ in the OR gate to give $\bar{A}+B$, and the inverter that follows returns

so the output is high only while $A$ is high and $B$ is low at the same instant. From the waveforms, $A$ is high over $0$ to $2$ s and over $3$ to $5$ s, while $B$ is high over $1$ to $2$ s and over $3$ to $4$ s. Both conditions hold together over $0$ to $1$ s and over $4$ to $5$ s. The output is a pulse from $0$ to $1$ s, zero from $1$ to $4$ s, and a second pulse from $4$ to $5$ s.