Find the correct combination of A , B , C and D inputs which can cause the LED to glow.

Step-by-step solution

The LED will glow if it is forward biased, which means the potential at the p-side must be higher than the potential at the n-side. Let Y₁ be the output of the upper NAND gate (p-side) and Y₂ be the output of the lower NOR gate (n-side). For the LED to glow, we need Y₁ = 1 and Y₂ = 0 . The upper circuit consists of two NOT gates (NOR gates with shorted inputs) followed by a NAND gate. Output of first NOT gate is A , and second is B . Y₁ = A B = A + B = A + B (using De Morgan's Law). The lower circuit is a NOR gate