When a ray of light enters a glass slab, the correct option is 3: the frequency does not change.

Option 1 states that only the frequency changes when a ray of light enters a glass slab. This is incorrect because frequency is a characteristic of the light wave itself and does not change when it enters a different medium.

Option 2 states that both the frequency and velocity change when light enters a glass slab. While the velocity does change, the frequency remains constant.

Option 3 is the correct answer. The frequency of the light wave does not change when it enters a glass slab. However, other properties such as the velocity and direction of propagation may change.

Option 4 states that both the frequency and wavelength change when light enters a glass slab. This is incorrect because the wavelength remains constant. The frequency is inversely proportional to the wavelength, so if the frequency does not change, neither does the wavelength.

In summary, when a ray of light enters a glass slab, the frequency of the light wave remains constant, while other properties such as velocity and direction may change.