Final answer:
Doubling the hydrogen chloride concentration leads to an increase in the number of collisions and subsequently an increase in the rate of the forward reaction.
Step-by-step explanation:
When the hydrogen chloride concentration doubles, the number of collisions between the reactants increases, which causes the rate of the forward reaction to increase. The correct answer is A) increases, increase. This is because the rate of reaction is directly proportional to the number of successful collisions per unit time. Therefore, doubling the concentration of hydrogen chloride results in a doubling of the number of collisions, assuming that the proportion of successful collisions remains the same.