The equation for speed (c) with relation to wavelength (λ) and frequency (ƒ) is:
c = ƒ x λ
ƒ = c/λ
Speed of red light in glass = 2c/3
Now, wavelength is directly proportional to speed. Since speed decreases, wavelength also decreases by by same amount.
Wavelength of red light in air = λ
Wavelength of red light in glass = 2λ/3
Frequency = (2c/3) ÷ (2λ/3) = 2c/3 x 3/2λ = 6c/6λ = c/λ
Since blue light refracts more than red light, speed of blue light in glass must be smaller than speed of red light in glass.
c (blue light) < c (red light)
c (blue light) < 2c/3
Hence, C is the answer