His notes followed a set of formal, rather old-fashioned rules.a. His notes were proper. b. His notes were correct.c. His notes were accurate.​

The phrase "His notes followed a set of formal, rather old-fashioned rules" suggests that his notes adhered to traditional standards that might be considered somewhat outdated or not in line with contemporary informal styles. The key terms here are "formal" and "old-fashioned rules," which imply an adherence to a structured and possibly conservative convention.

Based on this understanding: a. His notes were proper. - This is the closest in meaning because "proper" suggests that the notes met the expected standards of formality and traditional correctness, which aligns with the idea of following "formal, rather old-fashioned rules."

b. His notes were correct. - This statement may be true, but it's not specific enough to capture the nuance of the original sentence. "Correct" just means there were no errors, without additional context on formality or adherence to any old-fashioned rules.

c. His notes were accurate. - Similar to "correct," "accurate" refers to the precision and truthfulness of the content. However, it does not specifically imply anything about the formality or traditionalism of the rules followed.