Answer:
c. enzyme activity is usually least active below its optimum pH
Explanation:
Enzyme activity depends highly on pH. Enzymes are biological molecules which are proteins and influence the rates of important biochemical reactions in the human body.
Enzymes has a value of pH at which it works most perfectly. This pH value is known as its optimum pH. Above or below this value, the enzyme does not function optimally.
Hence, a change in pH usually produces a change in enzyme activity because enzyme activity is usually least active below its optimum pH