Right choice is (d) HU
Explanation: HU stands for Hounsfield Unit. It represents the opacity to the X – rays. Higher the opacity, the brighter the image is formed on the screen. A normal X-Ray has the bones and tissues blocking its path so the opacity is really low but with CT Scan, with its 3D imaging technique, the opacity for the tissue is increased and the image is taken better. The CT is able to negate out the resistance provided by the bone and other tissues.