Which one of the following statements concerning the General Public License (GPL) is true?

The core purpose of the GPL is to use copyright law to ensure software freedom - specifically that source code must be made available to anyone who receives the binary, and that derived works must also be distributed under the GPL. This 'copyleft' mechanism prevents the source from being made proprietary.

The GPL and BSD licenses are distinctly different; BSD is a permissive license that allows incorporation into proprietary software, while the GPL requires derivative works to remain open source.

GPL software may be sold for money; the GPL permits commercial distribution as long as the source code is also made available to recipients.

The GPL does not require sending changes to the original author; it requires making source available to whoever receives the binary, but there is no obligation to contribute back upstream.

The GPL contains no clauses restricting use in safety-critical systems like nuclear reactors or air traffic control; such restrictions are found in other licenses but not the GPL.