What term describes an enzyme's ability to act on a specific substrate only?

The correct answer is 'Specific.' This term defines the ability of an enzyme to act on a particular substrate due to the unique shape and chemical properties of the enzyme's active site, which allows it to bind only to specific substrate molecules. This specificity is crucial for the enzyme's function, as it ensures that only the intended biochemical reactions occur within the organism.

Enzymes have specific structures tailored to fit their corresponding substrates, much like a key fits a lock. This specificity is essential for maintaining metabolic control in biological systems, as enzymes are involved in catalyzing reactions that are vital for life. As such, the enzyme's action on a specific substrate helps regulate various biochemical pathways and prevents unwanted reactions.

The other terms do not accurately convey this concept. A competitive scenario refers to a type of inhibition where different molecules compete for the same active site, thus affecting the enzyme's activity without addressing its specificity. The term 'variable' implies changes can happen and does not relate directly to the enzyme's interaction with substrates. 'Active' generally describes an enzyme in its functional state but does not encompass the essence of substrate specificity.