"went off a tangent" is an idiomatic expression that means deviating from the main subject or topic of discussion and starting to talk about something unrelated or off-topic. The sentence suggests that the speaker started discussing matters that were not directly related to the main point or focus of the conversation. Option C, "began to discuss unrelated matters," is the correct option because it shows that the speaker veered away from the main topic and started talking about things not originally part of the discussion.