Concept:Transistors are semiconductor devices that serve mainly to increase the strength of weak electrical signals.Explanation:A transistor operates similarly to a triode valve by controlling a larger current using a smaller input signal.Its primary function is to amplify signals, meaning it increases the amplitude of voltage, current, or power.Rectification and a.c./d.c. conversion are functions of diodes and other devices, not transistors.Answer:C. amplification of signals