Select the option that can be used as a one-word substitute for the given group of words.
To protect from harm or danger

Let's evaluate the given options:
Defend: To protect someone or something from harm, danger, or attack.

Pretend: To behave so as to make it appear that something is the case when in fact it is not.

Guide: To show the way to others.

Advise: To offer suggestions about the best course of action.

Thus, Defend is the appropriate one-word substitute.