E8C04:What technique minimizes the bandwidth of a PSK31 signal?

Direct answer summary (2024-2028 NCVEC pool)

The official correct answer for E8C04 is Use of sinusoidal data pulses.

The key fact is Use of sinusoidal data pulses. Match that fact to the exact condition in the question; the other choices describe a different rule, band, component, or operating situation.