Of the options listed below, the speed of sound is highest in _____ at 25°C.

Aluminium

The correct answer is option 2. Aluminium.

Out of the listed options, the speed of sound is highest in Aluminium at 25°C.

Sound travels faster through denser and more rigid mediums.

Solids typically have higher densities and rigidity compared to liquids and gases.

Among the given choices, Aluminium is a solid metal, while Ethanol is a liquid and Oxygen and Hydrogen are gases.

Therefore, sound will travel fastest in Aluminium at 25°C.