d) 244 Pu 94
Primarily, stability is based off of the ratio of neutrons and protons in the nucleus. For smaller elements (20 protons or less) a 1:1 ratio is considered stable. For larger atoms (greater than 20 protons), more neutrons are required to stabilize the nucleus.
Plutonium 244 has a proton to neutron ratio of 94(p) : 145(n)
Plutonium has a number of isotopes, but all 15 isotopes are radioactive because to some degree they are unstable and as a result, decay, emitting particles and some gamma radiation as they emit. Plutonium readily undergoes fission, meaning that when it is hit with particles such as neutrons, it will split and release energy. Plutonium is one of the most common isotopes used in nuclear weapons for this reason. Plutonium-244 is actually the most stable isotope of plutonium but is still considered radioactive.
Therefore, 244 Pu 94 is radioactive