Which best describes the purpose of a controlled experiment
describes the purpose of a controlled experiment?
to make sure the results of the experiment support the hypothesis
to prevent a dangerous condition resulting from a reaction
to examine whether one variable causes a change in another
to prevent an undesired change in a variable

The purpose of a controlled experiment is to examine that whether the other variables causes a change in another.

The experimental setup of controlled experiments are designed to test the hypotheses. Usually there are one or more conditions which are considered as independent variables and measurements are taken which are called as dependent variables. It allows to change more than one variable, on the condition that all other variables are controlled. So, that it is easy to keep a track of changes or rather amount of changes is measured properly.

C to examine whether one variable causes a change in another

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.