In the experiment on tracing the path of a ray of light passing through a rectangular glass slab, the correct setting of the protractor (D), for measuring the angle of incidence (∠i), and the angle of emergence (​∠e), corresponds, respectively, to diagrams is 

(a) K and M                   (b) K and N                   (c) L and M                       (d) L and N

Dear Student ,
Here the option a is correct as the K and M are the correctly measured .Look here in this case angle of incidence must be equal to the angle of emergence .So here from these diagrams we see that the angle of incidence is equal to the angle of emergence is only in K and M diagrams .

Hope this clears your doubt .

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