If a low-level signal is placed at the input to a transistor and a higher level of the signal is produced at the output, what is this effect called?

Amplification is the process of taking a weak input signal and using a local power source to create a larger, stronger replica of that signal at the output. This is the primary function of a transistor when used in audio, intermediate, or radio frequency amplifier circuits. The distractors refer to entirely different signal processing techniques used in radio. Detection (or demodulation) extracts the audio or data from a radio wave, rectification changes AC to DC power, and modulation adds information to a carrier wave so it can be transmitted.