How does an increase in reactant concentration affect the rate of reaction?
it leads to more frequent collisions, which increase reaction rate.
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it leads to less frequent collisions, which increase reaction rate.
it leads to more frequent collisions, which decrease reaction rate.
it leads to less frequent collisions, which decrease reaction rate.

It leads to more frequent collisions, which increase reaction rate

Explanation:

An increase in reactant concentration leads to an increase in reactant particles which increases the number of collision in a reaction and then causes a reaction to proceed faster.

This belief was first suggested by the collision theory which states that for a reaction to occur, the reacting particles must collide with one another. However, further studies show that not all collisions lead to a reaction, and only effective collisions lead to a reaction.

Hence, an increase in reactant concentration increases the frequency of collision which in turn increases the chances of effective collision and subsequently sincreases the reaction rate.

Increasing the concentration of reactants affects the rate of reactions by increasing the frequency of collision of reactant molecules. Thus, the rate of reaction is increased.

Generally, reactions occur when the molecules of reactants collide with one another. According to the theory of collision, the more the rate at which the molecules of reactants in a reaction collide, the more the speed of the reaction per unit time.

Also, the more the concentration of reactants, the more the molecules of the reactants in the reaction vessel. Thus, the rate of reaction is increased through more collisions of the molecules of reactants as the concentration of reactants increase.

More on rate of reactions can be found here:

the temperature of the gas, in k, not celsius.