Answer: X rays have a short wavelength. 
Explanation : 
The wavelength of the x rays is, λ₁ = 0.01 nm to 10 nm 
The wavelength of the visible light is, λ₂ = 380 nm to 750 nm 
The frequency is inversely proportional to the wavelength of light i.e. 
 
So, the high-frequency x-rays have a short wavelength as compared to visible light. 
Hence, the correct option is (d) " short ".