The correct answer is (c), violet.
Violet light has the shortest wavelength of visible light, and therefore travels the slowest in glass. Red light has the longest wavelength of visible light, and therefore travels the fastest in glass.
The speed of light in a medium is given by the equation $v = \frac{c}{\mu}$, where $c$ is the speed of light in a vacuum and $\mu$ is the refractive index of the medium. The refractive index of a medium is a measure of how much light slows down when it passes through the medium. The refractive index of glass is typically around 1.5, which means that light travels about 1.5 times slower in glass than it does in a vacuum.
The refractive index of a medium is dependent on the wavelength of light. Shorter wavelengths of light have a higher refractive index than longer wavelengths of light. This is why violet light, which has the shortest wavelength of visible light, travels the slowest in glass.