JEE Main 2019 Physics question

In a double slit experiment, when a thin film of thickness t having refractive index μ is introduced in front of one of the slits, the maximum at the centre of the fringe pattern shifts by one fringe width. The value of t is (λ is the wavelength of the light used):

• (A)
• (B)
• (C)
• (D)Correct