Direct sequence is used with which type of transmission?

Direct sequence is one of the two primary methods used in spread spectrum transmissions, alongside frequency hopping. It works by combining the data signal with a much faster bit sequence (a chipping code), intentionally spreading the RF energy across a wide frequency band to reduce interference and increase security. AMTOR, RTTY, and standard Packet radio are narrowband digital modes. They do not use high-speed spreading codes to distribute their signal energy across a wide spectrum, making 'Spread spectrum' the only correct answer.