Functional protein is

Enzymes are proteins which accelerate the rate of the reaction. The enzymes are functional protein as they play important role in metabolic reaction. Part of an enzyme where substrate molecules get attached is known as an active site. On attachment to the active site, the substrate is changed into the product. The substrate and product have different energy state. The energy gap between the substrate and the product is known as activation energy which is the minimum energy required to initiate a reaction. The enzymes lower the activation energy and lead to the faster conversion of substrate to the product.
So, the correct answer is option A.