The most powerful was economic prosperity of European countries. They could exploit the land and the people of Africa and take their raw resources such as diamonds for example and take them to Europe. There they would be able to make goods with them and sell them to people for a lot of money and this meant that they would grow vastly richer than they were before.
I believe A :) Good luck!
D) Great britain was weakened by WW2
C and D are wrong due to the fact that it does not occur in July and there is no direct sunlight. Also thing it’s B or A but I’m thinking its A due to the fact it doesn’t always occur on the 21st but also the 20th.