Final answer:
Mercury (Hg) is the element from the provided list that is a liquid at room temperature. Cesium (Cs) is solid at 25°C but melts just above this temperature, and bromine (Br2), although not listed, is also a liquid at room temperature.
Step-by-step explanation:
Among the listed elements, which are Al, Au, Br, Ca, Cs, F, H, He, Hg, I, K, Mg, Na, O, P, S, and Si, mercury (Hg) is the element that is a liquid at room temperature. While cesium (Cs) is a solid at 25°C, it is worth noting that it melts at a slightly higher temperature and thus is close to being a liquid at room temperature. Bromine (Br2) is also a liquid at room temperature, but it is not listed among the options provided to the student.
The alkali metals, which include cesium (Cs), are known for being soft and highly reactive but are solid just below room temperature. Other elements present such as calcium (Ca), sodium (Na), and potassium (K) are also solid at room temperature, as are most of the elements on the periodic table.