What is a transistor primarily used for?

A transistor is primarily used for controlling electrical signals, serving as a fundamental building block in electronic devices. It functions as a switch or an amplifier in circuits, allowing it to modulate the flow of electrical current. By adjusting the input voltage or current, a transistor can effectively control how much current flows through a circuit, which is essential in applications ranging from simple electronic switches to complex computer processors.

The other options, while relevant to different fields of science and technology, do not accurately represent the primary function of a transistor. Thermal conductivity relates to the ability of a material to conduct heat, storing energy typically refers to components like capacitors or batteries, and creating chemical reactions is more aligned with catalysts or reactants in chemistry rather than the purpose of a transistor.