When the police arrest someone today, they must follow certain agreed-upon procedures. they are said to be following
the rule of law.
the separation of powers.
due process.
the legislative branch.

Due process

Explanation:

Due process refers to the legal obligation of the government to respect people's rights and to carry out all legal proceedings following established rules and principles so that all accused people receive fair and non-arbitrary treatment and are subject to rules that are the same for everyone.

So, if the police (a source of authority) arrests someone, they must follow certain agreed-upon and non-arbitrary procedures, in order words, they must follow due process.

due process.

Explanation:

When the police arrest someone today, they must follow certain agreed-upon procedures. They are said to be following the due process.