Final answer:
The main difference between the Diamond DA-20 Katana and the Diamond DA-40 Diamond Star is the number of seats: the DA-20 has two seats, while the DA-40 has four.
Step-by-step explanation:
The main difference between the Diamond DA-20 Katana and the Diamond DA-40 Diamond Star is a) the number of seats. The DA-20 Katana is a two-seat aircraft often used for flight training and personal use, while the DA-40 Diamond Star is a four-seat aircraft that can also be used for training, personal use, and light transport. Another notable difference, though not the one specifically asked about, is that the DA-40 typically features a glass cockpit and has a larger engine than the DA-20.