The option that can be used as a one-word substitute for the given group of words "Someone who attacks cherished ideas or traditional institutions" is "Iconoclast."

1. Believer: A believer is someone who has faith or conviction in certain ideas, doctrines, or religious beliefs. They hold strong positive convictions rather than attacking cherished ideas or institutions.

2. Iconoclast: An iconoclast is a person who attacks or challenges established or cherished ideas, traditions, or institutions. They question and criticize prevailing beliefs or practices, often seeking to bring about change or reform. Iconoclasts are often seen as rebels or dissenters who challenge the status quo.

3. Humanitarian: A humanitarian is a person who is concerned about the well-being and welfare of others. They are driven by compassion and a desire to alleviate human suffering. While they may work to improve societal conditions, they do not necessarily attack cherished ideas or traditional institutions.

4. Misanthrope: A misanthrope is someone who dislikes or has a general distrust or contempt for humankind. They have a negative view of people and tend to avoid social interactions. While a misanthrope may hold a negative opinion of cherished ideas or institutions, they are more focused on their dislike of people in general rather than actively attacking those ideas or institutions.

Therefore, in the given context, the word that can be used as a one-word substitute for "Someone who attacks cherished ideas or traditional institutions" is "Iconoclast," as it specifically refers to individuals who challenge or attack established beliefs or institutions.