Final answer:
The Diamond DA-40 Diamond Star's distinctive feature compared to the DA-20 is its larger side windows which provide pilots with better visibility and more natural light in the cockpit.
Step-by-step explanation:
The distinctive feature of the Diamond DA-40 Diamond Star as compared to the DA-20 is c) Larger side windows. Unlike the DA-20, which has smaller side windows, the DA-40 features expanded side windows that offer pilots better visibility and more natural light within the cockpit. This design element contributes to an overall feeling of spaciousness and provides pilots with enhanced situational awareness during flight.