Which event below accurately reflects the reason great britain joined the allied forces? austria-hungary's declaration of war on serbia italy's decision to join the central powers germany's infiltration of belgium germany's invasion and attempt to colonize north africa

Well by reading the part "Allied forced" I'm going to assume this talk about WW2. 
The first answer is about WW1, so that's obviously incorrect.
I'm not sure where the central powers come from, but I know during WW2 they joined the Axis Powers. So this is incorrect.
This sounds more like it's from WW2. Once Germany managed to invade Belgium, and other countries like Poland, that's when the allied powers started forming together. That's when Britain I believe stopped trying to "appease" Hitler. by letting him invade other countries and take control. This is correct.
The last one is most likely talking about when European countries scrambled to take control of the many countries in Africa during the 1880s. This doesn't relate to WW2, so this is incorrect.