a b c d

c

$ \text{n fringe width of red light = n+1 fringe width of blue light}$ $ \Rightarrow n\lambda_r = (n+1)\lambda_b$ $ \Rightarrow 7.8 n = 5.2 (n+1) $ $ \Rightarrow 2.6 n = 5.2 $ $\Rightarrow n = 2$