Sound Velocity is Maximum In

Sound Velocity is maximum in a solid medium because medium particles vibrate back and forth and the vibration is transmitted to the next particle whereas the sound waves propagate from each medium. 

The formula for Sound Velocity in a Medium is:

 v=γRTM−−−−−√ where R stands for universal gas constant, T denotes temperature, and γ refers to adiabatic gas constant. 

Therefore, the velocity of sound is inversely proportional to the molecular mass of the gaseous medium in which it is traveling in. We are provided 4 gases out of which oxygen is the heaviest with a molecular mass of 16u and hydrogen gas is the lightest with a molecular mass of 1u hence the sound velocity will be maximum in the hydrogen gas medium.