Question:

In the given logic circuit shown in the figure, inputs $ A $, $ B $, $ C $, and $ D $ are applied as shown. An LED is connected at the output. In which of the following combinations will the LED glow?

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To solve logic circuit problems, break down the circuit step by step and apply the appropriate truth tables for each gate.

Updated On: Jan 23, 2026

$ A = 1 $, $ B = 1 $, $ C = 0 $, $ D = 0 $
$ A = 1 $, $ B = 0 $, $ C = 0 $, $ D = 0 $
$ A = 0 $, $ B = 1 $, $ C = 0 $, $ D = 0 $
$ A = 1 $, $ B = 1 $, $ C = 1 $, $ D = 1 $

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The Correct Option is D

Solution and Explanation

Step 1: Analyze the logic gates.
In the given logic circuit, $ A $ and $ B $ are inputs to an AND gate, and the output of this AND gate is input to another gate along with $ C $ and $ D $. The LED will glow when the output of the logic circuit is high (i.e., when the combined output of the gates is 1). Step 2: Apply the logic gate operation.
After applying the truth table for the AND and OR gates in the circuit, we find that the LED will glow when $ A = 1 $, $ B = 1 $, $ C = 1 $, and $ D = 1 $. Step 3: Conclusion.
Thus, the LED will glow in option (4). Final Answer: \[ \boxed{A = 1, B = 1, C = 1, D = 1} \]

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