(A) Enzymes are proteins that catalyze biological reactions. Thus, the option A is correct.
(B) Enzymes act efficiently at a optimum value of temperature and pH. Thus, option B is incorrect.
(C) Co-enzymes do not increase the activities of enzymes. They help enzymes in the catalytic activity. Co-enzymes bind to the active site of the catalyst and participate in the enzyme activity. Thus, option C is incorrect.
(D) Enzymes are highly specific in their action on substrates.
Thus, option D is correct.