The elements listed in the choices are radioactive materials. These materials are very unstable because of the high neutron-to-proton ratio in the nucleus. Because of this, they spontaneously undergo nuclear reactions by giving off sub-particles in order to lower the ratio. They emit radiation in the form of alpha, beta or gamma.
Half-life is the length of time it needs to completely react half of the original amount. For example, it would take Bismuth 60.5 seconds to use up half of its amount. Therefore, the lower the half-life the faster the reaction is and the lesser the residual amount will be. With that being said, the answer would be Bismuth because it only takes seconds.
