Which of the following digital modes is designed for meteor scatter communications?
Why Meteor scatter bounces signals off the ionized trails left by meteors, which last only fractions of a second, so the mode must pack a whole message into a very short burst and repeat it rapidly. MSK144, part of the WSJT-X suite, does exactly that: it sends messages in 72 ms frames repeated continuously within 15-second sequences, so any brief ping can carry a complete decodable transmission. The "144" in the name is a reminder that it is used mainly on 6 meters and 2 meters where meteor scatter works best.
Watch out WSPR is the other WSJT-family mode in the list, but it is a very slow, very weak-signal beacon mode for probing propagation paths, the opposite of a fast burst mode. Hellschreiber is an old facsimile-style text mode, and APRS is a packet-based position and data reporting system.
MSK144: Meteor Scatter Kilobits in 144 ms-ish bursts, used on 144 MHz. WSPR whispers slowly, meteors need speed.