Density altitude is the altitude in the standard atmosphere that would have the same air density as the air you’re actually experiencing. That’s why the statement describing density altitude as the altitude corresponding to current air density is the best choice. Air density is what affects engine power, prop thrust, and wing lift, so when the density altitude is higher than your true altitude (due to hot weather, low pressure, or high humidity), performance suffers; when it’s lower (cold air, high pressure), performance improves. It’s not simply the current altitude above sea level, nor the altitude where temperature is standard, nor the maximum altitude for climb.

Vapp is the approach airspeed you fly during the approach that is chosen to optimize glide performance. In practice, the speed is selected to give the best lift-to-drag ratio, which means the greatest glide distance for the aircraft in an engine-out scenario. This makes Vapp the speed you’d use to maximize how far you can glide from a given altitude while still being able to manage the approach safely. Relating it to Vref, Vref is the reference landing speed used to set the target on final approach in the landing configuration. Vapp is chosen with that reference in mind but is not the same as Vref; it is typically set to a value that provides the best glide efficiency (and thus energy management) while still allowing a stabilized approach and a safe margin above stall.

The key idea is matching speeds to the takeoff actions: when to rotate, and what speed means you can safely continue with an engine out. The rotation speed is VR, the speed at which you lift the nose to get airborne. For engine-out performance, you need to be able to maintain a safe climb with one engine inoperative, which is defined by V2, the takeoff safety speed. In addition, V1 is the decision speed: if an engine failure occurs before V1 you should abort the takeoff; if it happens at or after V1 you continue. So the speeds used to assess rotation and engine-out performance are VR for rotation, V2 for takeoff performance with an engine out, and V1 as required by the takeoff profile. Other options don’t match these roles: Vne/Vmo are maximum operating speeds, and Vs1 is the stall speed in takeoff configuration, not the rotation or engine-out performance speeds.

Length can be given in both metric and imperial units, and they describe the same dimension. The stated accurate length is 14.2 m with 46'8''; the true statement is the one that lists both of those values together, since it preserves the exact pairing provided. For a quick check, 46'8'' converts to about 14.22 m, which rounds to 14.2 m, so the two representations are consistent within typical rounding. The other options mix a different metric value or a different feet-inches value, so they don’t match the given length.