Why do DX stations often transmit and receive on different frequencies?

Operating "split" allows a popular DX station to listen on one frequency or a range of frequencies while transmitting on a single, clear frequency. This improves efficiency by preventing the massive "pileup" of calling stations from drowning out the DX station's own transmissions. Additionally, split operation separates the DX station from the pileup and accommodates regional band plan differences. For example, a DX station might transmit in a voice segment that is prohibited for US operators, so those operators must transmit on a different, authorized frequency while listening to the DX station on their transmitting frequency. Since all these operational, practical, and regulatory reasons are valid and commonly used to facilitate international contacts, all the provided choices are correct.