In a simple 2 stage CW transmitter circuit, the oscillator stage and the class C amplifier stage are inductively coupled by a RF transformer. Another role of the RF transformer is to:

In a basic two-stage CW transmitter, the RF transformer linking the oscillator to the amplifier does not simply pass the signal along. Its windings are typically resonated with a capacitor to form a tuned circuit. This tuned circuit selects the desired frequency of oscillation and provides vital impedance matching between the high-impedance oscillator output and the low-impedance amplifier input. Other options describe functions not performed by this interstage transformer. It does not provide the feedback required for oscillation (that occurs internally within the oscillator circuit itself). Furthermore, it does not act as a balanced mixer because there are no multiple frequencies being mixed, and it is not a pi filter, which is generally a specific output network made of one series inductor and two shunt capacitors.