The moment when Ellen and Lochinvar mount his charger belong to the climax.
Climax is a structural part of a plot and is at times referred to as a crisis. It is a decisive moment or a turning point in a storyline at which the rising action turns around into a falling action.
The correct answer between all the choices given is the first choice or letter A. I am hoping that this answer has satisfied your query and it will be able to help you in your endeavor, and if you would like, feel free to ask another question.