a b c d

d

$\text{If Intensity of each wave is I then } I_0 = 4I$ $ \text{ At a point which is equidistant from slit the phase difference due to path difference is zero}$ $ \text{Net path difference at such point is } \frac{\pi}{3}$ $ \Rightarrow I_{net} = I + I +2I cos \frac{\pi}{3} = 3I = \frac{3I_0}{4}$