When a light ray enters a rectangular glass slab obliquely from air, what happens to its direction at the first interface?

The correct option is It bends towards the normal.

When a light ray travels from one medium to another, it changes its speed and direction due to a change in the optical density of the media. This phenomenon is known as refraction.

Air is an optically rarer medium, meaning it has a lower refractive index ($n_1$). Glass is an optically denser medium, which means it has a higher refractive index ($n_2$).

According to Snell's Law of refraction:
$n_{1}sin\left(i\right)=n_{2}sin\left(r\right)$

Since the light ray is entering glass from air:
$n_2>n_1$

Therefore, for the equality to hold true, the angle of refraction ($r$) must be smaller than the angle of incidence ($i$):
$r<i$

Because the angle of refraction is smaller than the angle of incidence, the light ray bends towards the normal at the first interface (air-to-glass boundary) of the rectangular glass slab.