The correct answer to the question: What is made up from crystalline solid that conducts a current only under certain circumstances, would be: a semiconductor detector made up of crystals.
Explanation:
Because crystals are pretty good insulators, but their electrons stored inside a crytalline tube, like is the case of these types of solid-state detectors, can be excited to react to certain types of stimulation, they are used as well. However, one thing about them is that they can only be made to conduct currents of electricity under certain excitation conditions that force the electrons to move from one side of the electrode, to the other. This is why the answer given above is correct.
