The correct answer is False.
Numerical aperture (NA) is a measure of the light-gathering ability of an optical system. It is defined as the sine of the half-angle of the cone of light that can be accepted by the system. In the case of a step-index fiber, NA is not constant. It depends on the refractive indices of the core and cladding materials, as well as the diameter of the fiber.
The refractive index of a material is a measure of how much light bends when it passes from one material to another. The refractive indices of the core and cladding materials are typically chosen so that light is guided down the center of the fiber by total internal reflection. This means that the light is confined to the core and does not escape into the cladding.
The diameter of the fiber also affects NA. A larger fiber has a larger NA than a smaller fiber. This is because a larger fiber can accept a larger cone of light.
In conclusion, NA is not constant in the case of a step-index fiber. It depends on the refractive indices of the core and cladding materials, as well as the diameter of the fiber.