What allows the transmitter to radiate a large number of discrete frequencies over a relatively wide band?

The frequency synthesizer is a crucial component in communications systems that allows the transmitter to radiate a wide range of discrete frequencies. By combining various techniques and technologies, a frequency synthesizer can produce multiple output frequencies from a single reference frequency. This capability is essential for modern radar and communication systems, which need to operate over diverse frequency bands to effectively perform their functions.

The frequency synthesizer usually employs phase-locked loops (PLLs) that enable precise control of frequency output. This flexibility makes it possible to tune into different frequencies without the need for physically changing components, thus improving response time and performance.

In contrast, the other options serve different roles within the transmission system. For example, a hybrid ring is typically used in signal routing and mixing but does not itself generate frequencies. A modulator is responsible for altering signals to carry information but does not directly generate or radiate discrete frequencies. A waveguide is a structure that directs electromagnetic waves, but it does not produce frequencies like a frequency synthesizer does. This is why the frequency synthesizer is the correct choice for allowing the transmitter to radiate a large number of discrete frequencies over a relatively wide band.