Answer:
Number of cells killed at the end of the experiment
Step-by-step explanation:
In research, the dependent variable is one whose value is dependent on another variable - a variable that is supplied by the researcher during the course of the experiment which is also known as the independent variable.
In this case, since the main objective of the research was to determine the most effective antibiotic against the E. coli, the variable that is supplied by the researcher (the independent variable) is, therefore, the different antibiotics while the variable whose value varies based on which of the antibiotics is introduced is the number of dead bacterial cells.
Therefore, the dependent variable is the number of cells killed by each antibiotic.