When monocromatic orange light is used instead of violet light in a convex lens, its focal length will:

increase

The correct answer is Option (1) → increase

Since, the refractive index is wavelength dependent, for shorter wavelength (like violet), the $μ$ is higher while for larger wavelength (like orange), the refractive index is lower.

and,

$μ∝\frac{1}{f}$

Hence, when using monochromatic orange light, instead of violet light, the focal length of the lens increases.