A small spherical water drop is suspended in the air, and a beam of light is horizontally incident. Why are the refraction angles of A light, B light and C light from water to air equal?

Because the first incident angle from air to water is the same, the second incident angle from water to air is the same (the two base angles of an isosceles triangle are equal). According to the refraction theorem, the second time has the same incident angle (the same incident angle).