Final answer:
Increasing the temperature and pressure increase the number of particles available to react, while decreasing the concentration does not. Therefore, the correct option is 3) Decreasing the concentration.
Step-by-step explanation:
Increasing the temperature does increase the number of particles of a substance available to react in a chemical reaction. When the temperature is increased, the reactant particles move faster and collide more frequently, resulting in an increased reaction rate.
Increasing the pressure also increases the number of particles available to react. This is because an increase in pressure forces the gas particles closer together, increasing their concentration and resulting in more collisions between the particles.
Decreasing the concentration of a substance does not increase the number of particles available to react. When the concentration is decreased, there are fewer particles present in a given amount of space, leading to fewer collisions between the particles.
Therefore, the correct option is 3) Decreasing the concentration.