Why Split operation solves several problems at once. Band allocations differ by country, so a DX station may be transmitting in a segment where the stations calling it are not permitted to transmit (for example a US phone segment versus the wider allocations in other countries). Splitting also moves the pileup off the DX station's transmit frequency so callers do not bury the station they are trying to hear, which keeps interference down and lets the DX operator work more stations per hour. Because each of those reasons is valid, the all-inclusive choice is correct.
Watch out Each individual statement is true on its own, so picking just one, such as reducing interference, misses the other equally correct reasons the pool lists.
Split = legal segments + clear pileup + faster rate. Three good reasons, so pick 'all of these'.