Final answer:
Mercury (Hg) is the only element in the listed options that is not a solid at room temperature; it is a liquid.
Step-by-step explanation:
The element that is not a solid at room temperature among the options provided is Hg (mercury). All other given elements, including Ag (silver), Au (gold), Mn (manganese), and Mg (magnesium), are in a solid state at room temperature. Mercury is unique because it is one of only two elements that are in a liquid state at room temperature, with bromine being the other non-metallic liquid element.