The correct answer is B. Brackets signal to the reader that words from a direct quotation have been changed by the writer.
Consider the following example: "Doctor Smith argued that it is common to experience headaches while on the medicine." Let's say a student wants to include that quotation in a paper but thinks it would be clearer with the addition of a word or two. Simply adding those words in brackets will clarify the quote to the reader AND will signal the fact that those words did not appear in the original.
Therefore, the student might instead write: "Doctor Smith argued that it is common [for these patients] to experience headaches while on the medicine." Now, the reader can tell that the words contained within the brackets are not part of Doctor Smith's original text.