A colour blind person can not differentiate between:

The correct answer is Option (3) → Red and green colour

Colour blindness is a sex-linked recessive disorder. It is caused by a defect in either the red or the green cone of the eye. So the person fails to tell red and green apart. The gene for it lies on the X chromosome. For this reason it is found in about 8 per cent of males but only about 0.4 per cent of females.

(1) Red and yellow colour — Can be told apart. Colour blindness does not affect this pair.

(2) Blue and green colour — Can be told apart. The blue cones are not defective in this disorder.

(3) Red and green colour — Cannot be told apart. The red or green cone is defective.

(4) Black and white colour — Can be told apart. Black and white are seen by the rods, which are normal.

NCERT Extract:
Mendelian Disorders – “… Myotonic dystrophy) (b) Autosomal recessive trait (for example: Sickle-cell anaemia) Colour Blidness : It is a sex-linked recessive disorder due to defect in 73 either red or green cone of eye resulting in failure to discriminate between red and green colour. This defect is due to mutation in certain genes present in the X chromosome.”