Final answer:
The result of Czar Nicholas' mobilization of his troops toward the German-Austrian border was a) Germany declares war on Russia
Step-by-step explanation:
After Serbia failed to meet the demands of the Austrian ultimatum, Austria-Hungary declared war on Serbia. In response, Russia ordered the mobilization of its troops. Germany, bound to support Austria-Hungary, declared war on Russia and ordered full mobilization.
This escalation of declarations of war ultimately led to the start of World War I, with Germany and Austria-Hungary comprising the Central Powers, and Russia, France, and eventually the United States forming the Allied Powers. Therefore the correct answer is a) Germany declares war on Russia