Final answer:
When every collision between reactants results in a reaction, temperature is the key factor that determines how fast the reaction occurs by affecting the frequency and energy of collisions.
Step-by-step explanation:
When every collision between reactants leads to a reaction, the factor that determines the rate at which the reaction occurs is temperature.
This is because the rate of a chemical reaction is affected by the energy of activation; however, if we are considering a situation where every collision leads to a reaction (which implies that activation energy is not the limiting factor), then temperature governs how often these successful collisions occur.
Increased temperature raises the kinetic energy of molecules, thereby increasing the frequency and energy of collisions, leading to higher reaction rates. Other factors such as concentration of reactants and the presence of a catalyst could also influence the rate of reaction, but temperature is the critical factor when all collisions result in a reaction.