Concept:A proposition is a declarative sentence that is either true or false, but not both. It must be universally agreed upon.Explanation:The sentence “Sum of 6 and 7 is 13” is a declarative statement. Everyone can check that 6 + 7 equals 13, so it is always true. It is not an open sentence (which uses variables), nor a definition (which explains a word), nor a conjunction (which joins two statements with “and”). Since it is a clear true/false statement, it is a proposition.Answer:B. Proposition