The confusion comes from the fact that the answer depends on the level of detail that you are going to account for. In vacuum, the speed of light is constant, regardless of frequency. That hold true, no matter how picky you are. Once the light moves in some matter, like air, water, etc., things change a bit. At a very high level, the speed in that media continues to be the same. Virtually all of intro physics rely on this being true.
Under closer scrutiny, that turns out not to be the case. There is a actually a tiny difference in the speed of light in media. The difference is very tiny and can be ignored most of the time. The phenomena is known as dispersion and accounts for some of the color fringes which you can see on highly magnified photos taken with bad lenses. For MCAT purposes, unless dispersion is explicitly mentioned in the question, you can safely ignore its existence.
A note on frequency/speed/wavelength. Once light is emitted, its frequency cannot be changed. As it transits through different media, its speed of propagation changes and the wavelength changes appropriately with that. The frequency remains the same.
