Optically active substances are those which
A. Produce polarized light
B. Rotate the plane of polarization of the polarized light
C. Produce double refraction
D. Convert a plane polarized light into circular polarized light

Hint: Optically active substances are those which react with the light passing through it and changes the optical properties of the light. The light which will be passed through it is not normal light. It is the plane polarized light and we will discuss the role played by optically active substances when plane polarized light is passed through them.

Complete step by step solution:
The magnetic field and electric field exist in the electromagnetic wave and light is also an EM wave and the direction of propagation of light and oscillations of magnetic field particles and electric field particles are mutually perpendicular.
All the particles in the light will be vibrating in all the directions. This kind of light is called an unpolarized light. When such unpolarized light is passed through the polarizer, the vibrations in all the directions will be hindered and vibrations along the plane of transmission axis of the polarizer will be present. Such light is called plane polarized light.
When such plane polarized light is passed through optically active compounds then the plane of polarization of that polarized light will be rotated.

Hence, option B is correct.

Note:
The interaction between optically active compound and plane polarized light causes the rotation of the polarization plane of the light. The direction of rotation of this plane determines the type of optically active compound. If the compound rotates the plane in clockwise direction then that compound is called dextrorotatory compound and if that compound rotates plane of polarization of polarized light in counter clockwise direction then that compound is levorotatory compound.