'a', 'b', and 'c' are all reasonable statements.
This is in the thermosphere which is at an altitude of 85-520km
Base SI unit is the unit that used for simple quantity like time=second and length= meter. They only have one unit.
Derived unit is more complex because you multiply or divide at least two base SI, making it have more than 1 unit. The example could be velocity which was time/length = m/s
