(A) Red
Red light bends the least while passing through a glass prism, meaning it travels fastest in glass. Since refractive index $n = c/v$, higher speed means lower refractive index. Hence, refractive index of glass is minimum for red light.
Key concept: Refractive index is inversely related to the speed of light in the medium. In dispersion, violet bends the most (highest refractive index) and red bends the least (lowest refractive index). Remember: VIBGYOR — refractive index decreases from Violet to Red.