The correct answer is c. 1st: Russia, 2nd: Germany, 3rd: France, 4th: Great Britain. Here is a chronology of these countries entering WWI:
On July 28th 1914, Austria declared war on Serbia. Austria had been a rival of Russia for a long time, as both competed for control in southeast Europe. Besides, Russia was allied to Serbia. Thus, when Austria declared war on Serbia, she was indirectly declaring war on Russia, so Russia started to mobilize troops on July 30th 1914.
On August 1st, Germany, allied of Austria, and Russia declared war on each other, thus both countries formally entered the war.
On August 3rd, Germany declared war on France, an ally to Russia. Then, France entered the war.
Finally, Germany invaded Belgium on August 4th. A treaty of 1839 guaranteed Belgium neutrality, and Britain was one of the guarantors. Consequently, when Germany invaded Belgium, a British demand was sent to Germany for assurance that Belgian neutrality would be respected. When the time limit ran out, Britain declared war on Germany and entered the war on August 4th.
