Which of the following best describes a mathematical theorem?

A mathematical theorem is defined as a proposition that can be rigorously proven based on previously established axioms and propositions. The key element of a theorem is the proof, which signifies that it is not simply an assertion or opinion but a statement supported by logical reasoning within the framework of mathematical logic. Theorems contribute to the progression of mathematics by building on foundational truths (axioms) and other proven statements, showcasing the importance of a structured approach to mathematical inquiry.

Other choices offer alternative descriptions that do not align with this formal definition. A statement based purely on opinions does not meet the rigorous standards of proof and verification that define a theorem. A simple mathematical operation refers to basic arithmetic or manipulative actions within mathematics but does not encompass the concept of a theorem. Lastly, a fundamental unanswered question in mathematics lacks the definitive proof that characterizes a theorem, placing it outside the realm of established mathematical knowledge.