Which word means to declare or affirm solemnly and formally as true?

The chosen word, "aver," specifically means to declare or affirm something as true in a formal or solemn way. This term is often used in legal contexts or formal statements, where one asserts the truth of a claim with confidence and responsibility. When someone avers something, they are not just stating it casually; they are presenting it with sincerity and certainty, often backed by evidence or conviction.

Other options present different nuances that do not align with the definition in question. "Acknowledge" typically involves recognizing the existence or truth of something but does not carry the formal affirmation aspect that "aver" does. "Advocate" refers to the act of supporting or recommending a cause or policy, which is unrelated to declaring something as true. "Temperate" describes moderation or self-restraint and relates to behavior rather than the act of affirming truth. Hence, "aver" stands out as the most precise choice regarding the formal declaration of truth.