Cuál es la característica principal para que un elemento sea radiactivo?

Idon't see the pic?

Answer isno products will be formedyou need the required activation energy/good geometry  to be able to get a reaction going. (without favorable  geometry, more energy would be needed.)this is because molecules in a reaction have electrons surrounding them. as molecules approach one another, they start to slow down because of the like charge repulsion (electrons have a negative charge—like charges repel). the appropriate amount of energy is needed to overcome that mutual repulsion. without that amount of energy, the speed of the molecules will be slowed down to such an extent  they do not even collide; they end up rebounding from each other.

Well what are the choices?