Final answer:
Decreased temperature is most likely to slow chemical reactions in cells by reducing the kinetic energy of molecules and altering enzymatic activity. The correct option is B.
Step-by-step explanation:
The environmental change most likely to slow chemical reactions in cells is decreased temperature. As the temperature decreases, the kinetic energy of molecules is reduced, which in turn slows down the rate of enzymatic reactions.
Enzymes have optimal temperature ranges for their activity, and temperatures lower than the optimum can cause changes in the enzymatic activity. In contrast, increased temperature typically speeds up reactions until a point at which proteins, including enzymes, denature.
An increase in the number of substrate molecules in solution usually increases the rate of reaction until the enzyme becomes saturated, while the neutralization of an acidic environment, if it was initially optimal for the enzyme, might lower enzyme activity but not as significantly as temperature changes. The correct option is B.