Answer:
c. The frequency of collisions between particles, the size of the particles.
Step-by-step explanation:
The reaction among 2 substances could speed up if the size of particles is smaller, for example it is easier to dissolve the common sugar than a cube of sugar, the reason is because since the size is smaller, the total area that is exposed to react is bigger.
Also, the frequency of collisions between particles, can increase the rate of reaction, because if they collide faster, the probability of react is bigger.