The correct option is A 1st August 1914
August, 1914. Actually the war begin on the last days of July, 1914. Tension began to prevail from 28th June 1914 when Serbian Nationalist assassinated Archduke Franz Ferdinand. And that time in Europe the major countries have a treaty among themselves as per that if one country declares war other also have to intervene. So Austria-Hungary & Germany paired up against Britain, France, Ireland & Russia.