Which punctuation is used to indicate an interruption in thought?

The correct choice is a dash. Dashes are specifically designed to indicate a break in thought or to introduce additional information that may disrupt the flow of a sentence. They can create a more dramatic pause than commas and often give the reader a sense of unexpectedness or surprise in the context of the text.

For example, in a sentence like, "I was going to—wait, what did you just say?—continue with my plan," the dashes clearly signify the interruption in the speaker's train of thought. This punctuation helps to convey the speaker's shift in focus and adds emphasis to the interruption.

In contrast, commas serve to separate elements within a sentence but do not create as strong an interruption. Periods indicate a full stop and cannot be used to signify an interruption within the same sentence. Semicolons are used to connect closely related independent clauses and, while they can indicate a pause, they do not denote an interruption in the same manner as dashes do. Therefore, dashes are the appropriate punctuation for showing interruptions in thought.