Final answer:
The most likely event a technician would observe when placing bacterial cells in a growth medium is cell division, as bacteria reproduce by binary fission in favorable conditions.
Step-by-step explanation:
When a lab technician places a colony of bacterial cells in a nutrient-rich growth medium, the event that is most likely to be observed under a microscope is A) Cell division. Bacteria, being unicellular organisms, grow by dividing. They do not grow by increasing in size like multicellular organisms. Instead, when they have acquired enough materials and have reached a certain stage in their cellular development, they undergo binary fission, which is their method of reproduction. Therefore, observing cell division is the most likely scenario in a growth medium that supplies ample nutrients for the bacteria to thrive.
Cellular respiration could occur but is not observable under the microscope as it's a biochemical process. Heat production and food storage are also processes that take place within cells, but they are not events that could be seen visually through a microscope.