B) eukaryotic cell because it's a key feature as a kingdom of organisms.
if I'm being honest I think it would be uhm C
Changing only one variable at a time is a rule for conducting a controlled
experiment.
Controlled experiment involves the use of dependent and independent
variables . Changes in the independent variable brings about a
corresponding change in the dependent variable which is then recorded.
It is however important to note that changing more than only one variable at
a time won't accurately determine if the changes in the results were as a
result of one cause. This is why only one variable has to be changed at a
time.
Read more about Hypotheses here brainly.com/question/12416923
It is called The Cell Theory.