Why FST4 (and its beacon-like cousin FST4W) was designed by the WSJT-X developers for the LF and MF bands, 2200 and 630 meters, where extremely narrow signals and long integration times pay off. It uses 4-tone Gaussian FSK, which shapes the tone transitions to keep the spectrum tight. It offers a menu of transmit/receive sequence lengths (15, 30, 60, 120, 300, 900 and 1800 seconds), and because tone spacing scales with the symbol rate, each of those seven period choices has its own tone spacing. Since all three statements describe the mode, the combined choice is correct.
Watch out Each individual statement is true on its own, so picking just the four-tone GFSK description or just the variable T/R periods leaves out equally valid characteristics.
FST4 = Four tones, and seven T/R periods each with its own spacing. Everything listed is true, so pick 'all'.