The correct option is A 28th July 1914
The assassination of the royal representative of Austria was an incentive for the nation to smother Serbia. Austria imposed unacceptable demands on Serbia. Serbia was given only 48 hours to fulfill those demands. At that time Russia supported and gave military help to Serbia. After getting support from Russia, Serbia denied the demands of Austria as a result the international conflict of the First World War got under way and Austria declared war against Serbia on 28th July 1914.