a personal idea must be identified with a\na. citation\nc. parenthesis\nb. letter from the author\nd. a new…

Brief Explanations:

In academic writing, when presenting a personal idea, it should be clearly distinguished from cited sources. A citation is used to identify ideas from other authors. A personal idea doesn't require a letter from the author, a parenthesis is mainly for in - text citations or clarifications, and starting a new paragraph doesn't identify it as a personal idea. A personal idea is simply presented as one's own thought and doesn't need a special marker like those for external sources. But among the options, when we want to make it clear that it's a personal thought and not a borrowed one, we don't use the elements for citing others.

Answer:

A. Citation