Colour blindness is due to defect in _________.

The correct option is A Cones

• Cones are the photoreceptor cells that contain iodopsin, porpyrosin and cyanopsin pigments. These cells are associated with the daylight vision and colour vision of eyes.
  
• Rods are the photoreceptor cells that contain rhodopsin pigment and associated with scotopic vision of eye i.e. vision under low light.
• Rhodopsin or visual purple (derivative of vitamin A) is a photopigment in the rods that are associated with scotopic vision of eye i.e. vision under low light or dim light.

Colour blindness is the inability to distinguish certain colours. It occurs due to defect in cones.

So, the correct answer is 'Cones'