What is keeping all variables the same except for the one being tested an example of?

A conclusion

A fair test

The engineering method

A good hypothesis

A fair test.

Explanation:

Hi, a fair test is used to do scientifically valuable experiments, is a controlled investigation to answer a scientific question.

In a fair test two or more things are compared.

It consists in changing only one factor (the one bieng tested) and keeping all the other conditions the same during an experiment.

The factor is called a variable.