Which of the following is characteristic of the FST4 mode?

FST4 is a digital mode specifically designed for weak signal communication on the LF and MF bands, such as 2200 meters and 630 meters. It was created to overcome the unique noise and fading challenges of these low frequencies. To achieve this, FST4 is highly adaptable. It utilizes four-tone Gaussian frequency shift keying (GFSK) to minimize bandwidth. It also allows operators to select from variable transmit/receive sequence lengths (ranging from 15 seconds up to 1800 seconds) and seven different tone spacings to optimize the signal based on current propagation conditions. Because all of the listed options accurately describe its highly configurable and specific technical design, "All these choices are correct" is the proper answer.