Answer:
D but don't be surprised if it is not the answer.
Step-by-step explanation:
This is a fascinating question. It caused quite a rubbarb in the 1960s when the question was debated by two different factions. One said that the stirrup made a huge difference to the society, and the other side said it did no such thing.
The answer you have to get rid of is B. Knights and others were riding horses before the stirrup was introduced. If think you are supposed to answer D, but it is not exactly correct either.
A is definitely not correct. A lance, a battle horse, and armor were very expensive. Only the knights could afford such things.
C the stirrup did no such thing. The age of knights went on for many years after the stirrup was introduced.
The problem with D is that the primary benefit of the stirrup had to do with the lance. The stirrup made it easier to stay seated in combat.
Since I have eliminated everything but D, you are sort of forced to pick D as the answer, but I believe it is a mighty poor one.