Sadly, we're forced to answer the question without the benefit of the
list of choices, which, for some reason, you decided not to let us see.
Index of refraction of a substance =
(speed of light in vacuum) / (speed of light in the substance).
Any number greater than ' 1 ' can be an index of refraction. A number
less than ' 1 ' can't be . . . that would be saying that the speed of light
in this substance is greater than the speed of light in vacuum.
