Catalyst is a substance which A. increases the rate of the reaction B. decreases the rate of reaction C. has no action on the rate of the reaction D. none of the above

The correct answer is A. A catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.

Option B is incorrect because a catalyst increases the rate of a reaction, not decreases it.

Option C is incorrect because a catalyst does have an effect on the rate of a reaction, it just doesn’t undergo any permanent chemical change itself.

Option D is incorrect because a catalyst does exist and does have the effect described in option A.