To increase the level of very weak radio signals from an antenna, you would use:

Signals received by an antenna are electromagnetic waves operating at Radio Frequencies (RF). Because these signals travel great distances, they are incredibly weak by the time they reach the receiving antenna and must be boosted before they can be processed by the receiver. An amplifier is an electronic circuit designed to increase the power or amplitude of a signal. Since the weak signal from the antenna is in the RF spectrum, an RF amplifier is required to perform this task. Other options are incorrect because oscillators generate their own signals rather than boosting external ones, and audio amplifiers are designed to work only with lower-frequency sound signals, not high-frequency radio waves.