Final answer:
The rate of reaction slows at 45°C primarily due to the denaturation of enzymes, which occurs when high temperatures disrupt the enzyme's active site and overall three-dimensional structure, inhibiting its ability to catalyze chemical reactions efficiently.
Step-by-step explanation:
The rate of reaction slows at 45°C due to the denaturation of enzymes, which is a process where enzymes lose their three-dimensional structure and, as a result, their function. This occurs because high temperatures affect chemical bonds within the active site of the enzyme, making it less suited to bind to substrates. As temperatures rise above an enzyme's optimum, the increased kinetic energy leads to a higher rate of molecular collisions, causing the structure of the enzyme, which relies on a precise conformation, to distort and lose its functional shape. Therefore, the correct answer to why the rate of reaction slows at 45°C is A. Denaturation of enzymes.