Explanation

(b) Absolute refractive index is defined as the ratio of speed of light in free space to that in a given medium
$ie,\ \mu = \frac{c}{v}$
For a given light, denser is the medium, lesser will be the speed of light and so greater will be the refractive index, $eg,$ as$\ v_{glass} > v_{water},\ \mu_{g} < \mu_{w}$. As speed of light in vacuum is always greater than speed in a transparent medium, so the refractive index in a transparent medium is greater than one.