What physical characteristic does a retinal scan biometric device measure?
a. the amount of light reaching the retina
b. the amount of light reflected by the retina
c. the pattern of light receptors at the back of the eye
d. the pattern of blood vessels at the back of the eye.

D)The pattern of blood vessels at the back of the eye.

Explanation: The retinal scan is the techniques which is used in the bio-metric mechanism.The scanning of the retina's blood vessel pattern is capture by the scanner is the unique technique which is used for bio-metric purpose.

Blood vessel pattern is capture because it is present in unique form for every person and will help in distinguishing a person from another person while scanning.

Other options are incorrect because light reflection or reaching near retina and light pattern are not the unique way for identification while scanning a person.Thus the correct option is option(D).