The manipulated variable is the one that the experimenter intentionally changes to observe its effect on the dependent variable. In this case, the manipulated variable is the ratio of fuel to oxygen. The scientist changes this ratio deliberately in each rocket launch.
The variable that should be controlled in all conditions is the one that the scientist wants to remain constant to isolate the effect of the manipulated variable. In this scenario, the scientist should control the amount of fuel or oxygen (depending on the specific condition being investigated). If the ratio is 2:1, 1:1, or 1:2, the total amount of fuel or oxygen should be kept constant for a fair comparison.
The responding variable is the one that changes in response to the manipulated variable. A reasonable responding variable for this experiment could be the height or distance the model rockets reach. The scientist can measure and compare the performance of the rockets to determine the impact of the different fuel-to-oxygen ratios on their flight characteristics.