a

$\text{At a point where the phase difference is }\Delta phi , \text{ The intensity at that point will be } I = 4I_0 cos^2{\frac{\Delta \phi}{2}} = 4I_0 \times 0.75$

$\Rightarrow cos^2{\frac{\Delta \phi}{2}}= \frac{3}{4}$

$\Rightarrow  cos{\frac{\Delta \phi}{2}}= \frac{\sqrt 3}{2}$

$\Rightarrow \frac{\Delta \phi}{2} = \frac{\pi}{6}, \frac{5\pi}{6}, \frac{7\pi}{6}, \frac{11\pi}{6}$

$\Rightarrow \Delta \phi = \frac{\pi}{3}, \frac{5\pi}{3}, \frac{7\pi}{3}, \frac{11\pi}{3}$

$\text{There is shift of 2 minima hence path difference should be greater than }3\pi$

$\text{Possible value of }\Delta\phi=\frac{11\pi}{3} , \frac{13\pi}{3} , \frac{17\pi}{3} ,  etc$

Correct option ia (a).