Final answer:
The correct chronological order of countries entering WWI is Austria-Hungary, Russia, Germany, France, and Britain. Option B is correct.
Step-by-step explanation:
The correct order in which nations entered World War I, from first to last, is Austria-Hungary, Russia, Germany, France, Britain. The conflict began when a Serbian nationalist assassinated Archduke Franz Ferdinand of Austria-Hungary, leading it to declare war on Serbia on July 28, 1914.
Russia mobilized in support of Serbia, which prompted Germany to declare war on Russia. Subsequently, Germany declared war on France, and when German troops invaded Belgium, Great Britain declared war on Germany, adhering to its treaty with Belgium.