Is an operating systems ability to be easily enhanced over time
a) performance
b) portability
c) reliability
d) extensibility

Extensibility

Explanation:

Extensibility is one of the features which can be easily enhanced over time. As the term extensibility refers to the addition of new features and adding more features to the present modules. This does not change the internal organisation of the earlier modules in fact they add new and more improved features to the present.

a few, because i should have plenty.