Correct Circuit:
$f = [\{(x_1x_2x_3)’ + x_4\}’ + x_3x_4]’ = [x_1’ + x_2’ + x_3’ + x_4 + x_3x_4]’ = [x_1’ + x_2’ + x_3’ + x_4]’ = x_1x_2x_3x_4’$
Faulty Circuit:
$f = [\{(x_1x_2x_3)’ + x_4\}’ + \mathbb{T}]’ = [\{(x_1x_2x_3)’ + x_4\}’ + 0]’ = x_1x_2x_3x_4’$
which is the same as the correct circuit.
$\therefore$ There is no way to detect the fault.
ANS: (D.)