In propositional logic, a compound proposition's truth value can be categorized. A tautology is always true, regardless of the truth values of its simple propositions. Conversely, a contradiction is always false. A proposition that is neither always true nor always false is known as a contingency.
Thus, Contingency is the correct answer. A contingency's truth value depends on the specific truth values of its components; it can be true in some cases and false in others. Option A, 'Equivalence,' describes when two propositions have the same truth value in all cases, not a type of proposition itself. Option B, 'Condition,' refers to an 'if-then' statement, which can be a tautology, contradiction, or contingency. Option D, 'Inference,' is the process of drawing conclusions, not a type of proposition.