The best definition for the word "enzyme" is option a) "a substance produced by a living organism which acts as a catalyst to bring about a specific biochemical reaction."
Enzymes are proteins that are produced by living organisms, such as plants and animals. They play a crucial role in biochemical reactions by acting as catalysts. Catalysts are substances that speed up chemical reactions without being consumed in the process. Enzymes are highly specific and can only catalyze specific reactions.
To better understand this, let's take an example. In the context of the life cycle of a plastic bottle, enzymes could be involved in the process of biodegradation. Biodegradation is the breakdown of materials by living organisms. Enzymes produced by microorganisms can break down the plastic bottle into smaller molecules, making it possible for the plastic to be recycled by nature.
In summary, enzymes are substances produced by living organisms that act as catalysts to facilitate specific biochemical reactions.