Final answer:
A holoenzyme is a combination of a protein and cofactors or coenzymes. These cofactors or coenzymes are necessary for the proper functioning of enzymes. The combination of the protein and the cofactors or coenzymes forms the active holoenzyme. The option (D) is correct.
Step-by-step explanation:
A holoenzyme is a combination of a protein and one or more substances called cofactors or coenzymes. Cofactors can be inorganic substances such as metallic ions (e.g. Zn²+ or Mg²+), while coenzymes are organic compounds, often derived from vitamins.
Both cofactors and coenzymes are necessary for the proper functioning of enzymes. The combination of the protein (apoenzyme) and the cofactors or coenzymes forms the holoenzyme, which is the active form of the enzyme. Therefore, option (D) is correct.