A biological catalyst is essentially(a) An enzyme(b) A carbohydrate (c) An amino acid (d) A nitrogenous base

The correct answer is (a) An enzyme.

Explanation: A biological catalyst is a substance that speeds up a biochemical reaction without being consumed in the process. Enzymes are the biological catalysts in living organisms. They are proteins that lower the activation energy required for reactions to take place, thus making metabolic processes occur faster and more efficiently.

Carbohydrates (option b) are a group of organic compounds used primarily for energy and structural purposes, not for catalysis.
Amino acids (option c) are the building blocks of proteins, including enzymes, but they themselves do not act as biological catalysts.
Nitrogenous bases (option d) are components of nucleic acids like DNA and RNA, and they have roles in genetic information storage and transfer, not catalysis.
Thus, the correct answer is (a) An enzyme.