Question

which factor causes the rate of an enzyme - catalyzed reaction to level off after initially increasing?
temperature decrease
ph level fluctuations
substrate absorption
products accumulating

Explanation:

Enzyme - catalyzed reactions initially increase in rate as substrate concentration rises. But when all enzyme active - sites are saturated with substrate (substrate absorption reaches a maximum), the rate levels off. Temperature decrease and pH fluctuations can affect enzyme activity but not in the way described here. Product accumulation can sometimes inhibit the reaction but not the main factor for leveling off the initial rate increase.

Answer:

substrate absorption