Which radioactive isotope would take the least amount of time to become stable? rubidium-91 iodine-131 cesium-135 uranium-238

The radioactive isotope that would take the least amount of time to become stable is rubidium-91. This is because this isotope is the most stable compared to the rest. This was determined by subtracting its atomic mass by its atomic number. The isotope with the least number of difference is the most stable, while the one with the greatest difference is the most unstable.

Difference:
Rubidium: 54 (most stable)
Iodine: 78
Cesium: 80
Uranium: 146 (least stable)

Kinetic energy = 1/2mv^2. since the mass is constant we can form a ratio of it's kinetic energy at 60 divided by that at 30 to see ke(60)/ke(30) = +(60^2)/30^2). this may be said to 300% increase.