Which event marked the beginning of the middle ages?
a. the renaissance
b. the crusades
c. the search for new trade routes
d. the collapse of the roman empire

It is generally agreed upon that "D. the collapse of the Roman Empire" marked the beginning of the Middle Ages, since this ushered in an era of lawlessness and economic depression. 

The event which marked the beginning of the Middle Ages was the collapse of the Roman Empire - D. With the Roman Empire declining more and more barbarian nations were starting to overwhelm the different cities and villages in Europe. This period is also often called the Dark ages.