Since it is round and it is the asteroid belt it is most likely that he found a dwarf planet
c. x-rays
My answer is that x-rays or gamma rays have the greatest (or highest) frequency waves.
According to Charles' Law the volume of an ideal gas is directly proportional to its absolute temperature in Kelvin keeping the pressure constant.
V∝ T, P is constant
where V, T and P are volume, temperature and pressure
= 
where V₁, T₁, V₂ and T₂ are initial volume, initial temperature, final volume and final temperature.
Every mole is 22.4 L at STP