The correct answer is D. Selenium.
A photoconductive cell is a device that converts light into an electrical signal. It is made up of a thin layer of selenium, which is a semiconductor. When light hits the selenium, it causes electrons to be released from the atoms. These electrons are then collected by an electrode, which creates an electrical current.
Quartz is an insulator, so it cannot be used in a photoconductive cell. Lithium sulphate is a salt, and barium titanate is a ceramic. Neither of these materials are semiconductors, so they cannot be used in a photoconductive cell either.
Selenium is a semiconductor, which means that it has electrical properties that are in between those of a conductor and an insulator. When light hits selenium, it causes electrons to be released from the atoms. These electrons are then collected by an electrode, which creates an electrical current. This current can be used to power devices such as light meters, cameras, and solar cells.