Answer:
(c) n1 = n2
Step-by-step explanation:
You want to know what can be said about the indices of refraction when a light ray continues on a straight path from one medium to another.
Refraction
When a wave passes through media where the index of refraction changes, its path will be bent in accordance with Snell's law. When the index of refraction does not change, the path will remain unbent.
The figure shows the path of the ray is a straight line, so we can conclude ...
n1 = n2 . . . . . choice C
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Additional comment
Even if the indices are different, the path will remain straight if the angle of incidence is 0—the path is normal to the interface between media. Here, the path is not shown as normal to the interface.