It's frequency remains unchanged but it's wavelength increases as velocity decreases (if in case light is moving from rarer to denser medium )
						

							It is clear that frequency of light would not change. Now depending weather light is moving from reader to sender or vice versa, wavelength would decrease in first case ,but would increase in second case.So it`s answer should be B and C.
						

							
The process refraction depicts the wave nature of light, which means it tells the dependency of light on velocity, wavelength, etc., 
 Since, 
               v=fλ
⇒    v ∝ λ  
⇒    if v increases then λ  increases and if v decreases then λ  decreases 
⇒    v is independent of f 
⇒   f remains unchanged in the process of refraction 
 
⇒  Since,
             vrarer > vdenser
Therefore, 
	
 on refraction from rarer to denser medium ⇒ wavelength of light wave decreases
	
 on refraction from denser to rarer medium ⇒ wavelength of light wave increases
in both cases frequency remains unchanged 
 
⇒ Correct Answer is (B)&(C)