Answer: b.

Explanation:
Underlying refers to something that lies beneath — the beliefs that underlie an argument, a motive that underlies an action, an issue that underlies a problem.

Underlining is something you do to make something stand out. File this one under eggcorn, I guess — another one that people hear and fail to realize is a separate word. Online, there do seem to be a number of people searching for “what does underlining problem mean.” No wonder they don’t understand.

Here’s Paul Brians’ explanation:

You can stress points by underlining them, but it’s “underlying” in expressions like “underlying story,” “underlying motive,” and “underlying principle.”

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