d
(d)Since \(P\) is ahead of \(Q\) by \(90^\circ\) and path difference between \(P\) and \(Q\) is \(\lambda /4.\)Therefore at \(A\), phase difference is zero, so intensity is \(4I\). At \(C \) it is zero and at \(B\), the phase difference is \(90^\circ\), so intensity is \(2I.\)