Which of the following HF digital modes uses variable-length character coding?

PSK31 utilizes a specific variable-length character coding system known as 'Varicode.' In Varicode, the most frequently used characters in English text (like the lowercase letter 'e') are assigned the shortest bit sequences, while less common characters have longer sequences. This allows PSK31 to maintain a reasonable typing speed despite its very narrow bandwidth. By contrast, RTTY uses a fixed-length 5-bit code called Baudot, which requires special shift characters to switch between letters and numbers. PACTOR and MT63 use different fixed-block or forward-error-correction encoding schemes rather than character-by-character variable-length coding.