When we have a mixture, at room temperature the hydrogen and oxygen gases, for example in the air, do not react to form water. First, because the activation energy of this reaction is very low. And second, because the movement of the molecules in their gaseous state is high, that is, they have a high kinetic energy. This kinetic energy is higher than activation energy, so the molecules don't react.
Therefore, the answer will be
c) the molecules collide with enough energy to overcome the activation barrier