HELP I know for sure its not C (someone already had asked this question and ALOT of people downvoted it so :/)
In the electron transport chain, ATP synthase is responsible for

producing a chemiosmotic gradient inside the matrix.
capturing oxygen and connecting it with free hydrogen ions
adding a phosphate group to ADP.
pumping hydrogen ions against concentration gradient.

A

Explanation:

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.