Correct answer: (1) Cell division is most rapid at 37^o C between 6 and 8 hours after it began.

A bacterial growth curve can be divided into four phases-lag, exponential, stationary, and death. During the lag stage, the bacteria are just introduced in the medium, which means the bacteria are still maturing and are not yet capable of division. When they start to divide, the exponential phase starts (steep slope), characterized by cell doubling. During this phase, the bacteria divide at a maximum rate until the nutrients become depleted. When the microbial count remains constant (horizontal line), it has reached the stationary phase. The last phase starts when the microbial count starts to go down again.

In the given bacterial curves, the exponential phase started at 2 hours after the culture was placed in the medium under 37^oC, and stopped dividing after the 10^th hour. Most rapid cell division occurred during the 6^th to 8^th hour, as illustrated by the steep slope of the curve.

Incorrect answers:

(2) Cell division is most rapid at 25^oC between 20 and 24 hours after it began.

At 25^oC, cell division started on the 2^nd hour and entered the stationary phase after the 16^th hour. The fastest cell division occurred during the 12^th hour, shown as the steepest slope of the curve.

(3) Cell division is most rapid at 18^oC between 4 and 8 hours after it began.

At 18^oC, cell division (exponential phase) started at the 2^nd hour and stopped at the 20^th hour. Fastest growth in number took place 14-16 hours after introduction of bacteria to the medium, shown in the curve as the steepest slope.

(4) Cell division occurs at the same rate no matter what the temperature.

This statement is false because the steepest slopes of the three curves are not located at the same part of the y axis (time).