The allied powers included

a. germany, austria-hungary, the ottoman empire, and eventually bulgaria

b. germany, austria-hungary, and eventually italy

c. france, great britain, germany, and eventually the ottoman empire

d. france, great britain, russia, italy, and eventually the united states

Reasons for joining WWI:Serbia (Dispute over Balkans)
Russia (Allied with Serbia)
Great Britain (Allied with Belgium)
France (Allied with Russia)
Belgium (Germany invaded to get to France)so the best answer is D

if you wanna know the Axis, and centeral here ya go

Reasons for joining WWI:Austria-Hungary (Dispute over Balkans)
Germany (Allied with Austria-Hungary)
Bulgaria (Allied with Germany)
Ottoman Empire (Allied with Austria-Hungary)

Neutral:Italy (Entered war in 1915 as an Allied Power)
United States (Entered war in 1917 as an Allied Power)
Spain
Netherlands
Norway
Sweden
Switzerland