In YDSE if a slab whose refractive index can be varied is placed in front of one of the slits. Then, the variation of resultant intensity at mid-point of screen with `mu` will be best represented by `(mu is greater than or equal to 1)

Explanation

`Delta x= (mu-1)t`, , For `mu=1, Delta x=0`
`:. I = "maximum" = I_(0)` , As `mu` increases path difference `Deltax` also increases. , or `Delta x = 0 to (lambda)/(2)`, intensity will decrease from `I_(0)` to zero. Then for `Delta x= (lambda)/(2) " to " lambda`, intensity will increase from zero to `I_(0)`. Hence option 3 is correct.