Explanation: Symbol 2, a circle with an arrow pointing out, is the standard block diagram representation for an oscillator or a signal source. A local oscillator (LO) is a circuit that generates a stable, continuous radio frequency (RF) signal. In receivers, this signal is mixed with the incoming RF signal to convert it to an intermediate frequency (IF). In transmitters, the LO often generates the carrier wave or a frequency used in frequency conversion stages. Symbol 1 typically represents an antenna or a general input/output connection. Symbol 3, a plain square/rectangle, is a generic processing block like a filter, mixer (if it has an 'X'), or other stage. Symbol 4, a triangle, universally represents an amplifier. Therefore, only symbol 2 accurately depicts a local oscillator.