A. 100 years is not correct

In modern era of European history, peace has not (yet) lasted for 100 years at a time. However, the continent has suffered under a Hundred Years War, which pitched England and France against each other.

In fact, there has been a recurring pattern of war and peace across the European continent for 600 years now. The Hundred Years War peaked in the early first half of the 15th century, focusing on France as a battlefield. It was only in 1429, when Joan of Arc led France's national revival, that the English were rolled back. The war finally ended in 1453.

B. 60 years is correct.

The major powers of Europe have not been at war with each other since May 1945, when the Second World War came to an end in Europe after six years.

That does not mean, however, that the continent has been free of war for the past 67 years (and counting). From 1991 to 1995, a series of fierce, but localized ethnic conflicts boiled over in the former Yugoslavia, most notably in Slovenia (in 1991), in Bosnia (1992-95) and Kosovo (1998-99).

If no major military confrontation breaks out in Europe until May 2045, about 33 years from now, then Europe will indeed have completed an entire century of peace.

C. 30 years is not correct.

From 1618 to 1648, during the Thirty Years War, it was Germany's turn as a battlefield. Religiously inspired wars reached well beyond its borders, though, and laid waste to much of the rest of Europe as well. The conflict only ended when the Peace of Westphalia, in 1648, finally ushered in a period of slow recuperation and brought more stability.

The 1700s brought the wars of the Spanish and Austrian Succession. They started just over five decades after the Peace of Westphalia and lasted for just over two decades — from 1701 to 1714 and then from 1740 to 1748. Late that same century, in the 1790s, France was torn apart by a bloody, but entirely domestic social revolution.

D. 20 years is not correct.

In the first few decades of the 20th century, it took less than 20 years — 19 years, 9 months and 21 days, to be exact — for the end of World War I, with hostilities ceasing on November 11, 1919, and the fragile peace that followed it to give way to the outbreak of World War II on September 1, 1939, with Germany's invasion of Poland.

The 19th century had started out on a war footing as well, when France's Napoleon Bonaparte set off in 1805 to conquer most of Europe. His victories included the defeat and conquest of the Austrian Empire in 1805 and Prussia in 1806. His string of military successes ended when he sought in vain to conquer Russia in 1812.