Which of the following terms signifies an act of disclosing or revealing something previously unknown?

The term that signifies an act of disclosing or revealing something previously unknown is "divulge." This word specifically implies the act of making information known that was not previously accessible or was kept confidential. When you divulge a secret or information, you are revealing details that others may not have been aware of before.

While terms like "expose," "announce," and "declare" also deal with revealing or presenting information, they don't capture the nuanced meaning inherent in "divulge." "Expose" often carries a connotation of unveiling something hidden, often with a focus on wrongdoing or scandal. "Announce" suggests a formal public statement of information but does not necessarily imply that the information was previously unknown. Similarly, "declare" typically conveys a strong statement or proclamation but does not explicitly indicate that the information was secret or unknown prior to the declaration. Thus, "divulge" is the most precise term for an act of revealing something for the first time.