1. **Problem Statement:** Identify which of the given sentences is not a proposition. 2. **Understanding Propositions:** A proposition is a declarative sentence that is either true or false, but not both. 3. **Analyze each option:** - A. "All birds are animals." This is a universal statement and can be true or false, so it is a proposition. - B. "4+0=5.1" This is a declarative statement that is false, so it is a proposition. - C. "$y^2 - 3 = (x - \sqrt{5})(x - \sqrt{5}), 4 \in \mathbb{R}$" This is a mathematical statement involving variables and a set membership. Since it contains variables without specific values, it is not a proposition because its truth value depends on $x$ and $y$. - D. "All integers $a$ are rational." This is a universal statement and true, so it is a proposition. 4. **Conclusion:** The sentence that is not a proposition is option C. **Final answer:** Option C is not a proposition.