The correct answer is C: Use of sinusoidal data pulses. The technique that minimizes the bandwidth of a PSK31 signal is the use of sinusoidal data pulses. PSK31 uses smooth, sinusoidal-shaped pulses instead of rectangular pulses, which reduces bandwidth. Rectangular pulses have sharp edges that create wide spectral sidebands. Sinusoidal pulses have smooth transitions that concentrate energy in a narrower bandwidth. PSK31's use of sinusoidal data pulses, combined with phase changes at zero crossings, keeps the signal bandwidth very narrow (about 31 Hz). This is why PSK31 can operate in such a narrow bandwidth - the smooth pulse shape minimizes spectral spreading. This is a key design feature that makes PSK31 bandwidth-efficient.