The official correct answer for G8A12 is Modulation in which digital data is transmitted using 0-, 90-, 180- and 270-degrees phase shift to represent pairs of bits.
The key fact is Modulation in which digital data is transmitted using 0-, 90-, 180- and 270-degrees phase shift to represent pairs of bits. Match that fact to the exact condition in the question; the other choices describe a different rule, band, component, or operating situation.