You construct a simple DC power supply using a transformer, rectifier and filter capacitor. If you use the supply to power a CW transmitter, what problem with signal quality could it cause?

A simple power supply consisting only of a transformer, rectifier, and filter capacitor lacks a voltage regulator. When a CW (Continuous Wave) transmitter is keyed, it suddenly draws a heavy load of current, which causes this unregulated DC voltage to sag or drop significantly. This sudden drop in operating voltage can cause the transmitter's internal oscillator to slightly change frequency during the key-down period. This unintentional, rapid shift in frequency sounds like a bird's chirp on the receiving end, hence the term "chirp." Other options are incorrect for this scenario. Key clicks are caused by keying envelopes that rise or fall too sharply, rather than by voltage sag. Overmodulation applies to voice or audio modes, and harmonics are multiples of the fundamental frequency, not a direct result of unregulated power supply voltage.