A few bacteria are placed in a nutrient solution. after several hours thousands of bacteria are present. which life activities are primarily responsible for this?

The answer is "Digestion and Reproduction"

When placing few bacteria on a nutrient solution, and for several hours it multiplies. The activities which take place in the scenario is the digestion and reproduction. Digestion occurs when the bacteria fed on the nutrient solution and Reproduction occurs when the bacteria gives birth to new bacteria.