a
(a)\(P\) is the position of \(11^{th}\) bright fringe from \(Q\). From central position \(O, P\) will be the position of \(10^{th}\) bright fringe.
Path difference between the waves reaching at \(P = \)\(= S_1 B = 10\)\(\lambda = 10 × 6000 ×10^{-10 }\)
\(10^{-10}\) \(= 6 \) \(\times\) \(10^{-6}\)\(m.\)