Which of the following narrow-band digital modes can receive signals with very low signal-to-noise ratios?
Why FT8 is part of the WSJT-X weak-signal family, using 8-tone FSK in about 50 Hz of bandwidth with 15-second transmit/receive sequences and heavy forward error correction. That combination of narrow bandwidth, long integration time and coding lets the decoder pull callsigns and grid squares out of signals around -21 dB signal-to-noise ratio referenced to a 2500 Hz bandwidth, well below what the ear can hear. That is why FT8 works routinely with low power and compromise antennas.
Watch out MSK144 is also a WSJT-X mode but it is optimized for meteor scatter, sending very fast repeated messages to catch brief, relatively strong pings rather than digging into the noise; AMTOR and MFSK32 are keyboard/text modes that need a usable signal-to-noise ratio to copy.
FT8 = "Faint Transmissions": decodes about 21 dB below the noise. If the question says weakest signals, answer FT8.