Which operating system is open source?

The most recent version of Windows (as of 9/2019) is

The most recent version of macOS (as of 9/2019) is

Android is a mobile operating system created by

iOS is a mobile operating system developed by

The most popular mobile operating system in the world is

This type of software is available freely to anyone to modify, study, and share

This type of software is privately owned and cannot be freely modified by anyone

Which is an example of open source software?

Which is an example of closed source (proprietary) software?

Which is an example of a word processing software?

Which application can be used to manage spreadsheets?