Choose the most suitable one-word substitution for the following.

A person who flatters powerful people in order to gain advantage.

The correct answer is Toady.

Explanation:
A toady (also known as a sycophant or flatterer) is someone who behaves in an overly subservient or obsequious manner toward powerful or influential people specifically to gain a personal favor, privilege, or advantage.

Let's analyze the meanings of all the given options:

1. Toady: A person who flatters powerful people in order to gain advantage. This directly matches the given definition.
2. Iconoclast: A person who attacks or opposes cherished beliefs, traditional customs, or established institutions.
3. Hermit: A person who lives in isolation or solitude away from society, often for personal or religious reasons.
4. Dogmatist: A person who asserts their own opinions or beliefs strongly and dogmatically, refusing to consider alternative views.

Thus, the most suitable one-word substitution for the given phrase is Toady.