Which best explains why Germany sided with Austria-Hungary following the assassination of Archduke Ferdinand?
The countries shared a border.
The countries had an existing alliance.
The countries were blamed for his death.
The countries both wanted to build colonial empires.

The countries had an existing alliance.

Explanation:

Germany gave blank cheque assurance to whatever step Austria-Hungary takes against Serbia even if Russia intervenes, Germany was ready to offer its help. Historians have referred to as the carte blanche or “blank check” assurance which marked a decisive moment in the chain of events leading  to the outbreak of the First World War in Europe.

Arch Duke Ferdinand was the Emperor of Austrian Hungary who was assassinated by a Serbian Nationalist on his visit to Bosnia. This demise shook the emotions of Wilhelm Kaiser of Germany who had just visited Archduke Ferdinand just two weeks back and there were a cordial ties between Germany and Austria-Hungary.

Germany sided with Austria-Hungary following the assassination of Archduke Ferdinand-The countries had an existing alliance

Explanation:

pepin the short[a] (german: pippin der kurze, french: pépin le bref, c. 714 – 24 september 768) was the king of the franks from 751 until his death. he was the first of the carolingians to become king.[b][2]

the younger son of the frankish prince charles martel and his wife rotrude, pepin's upbringing was distinguished by the ecclesiastical education he had received from the monks of st. denis. succeeding his father as the mayor of the palace in 741, pepin reigned over francia jointly with his elder brother carloman. pepin ruled in neustria, burgundy, and provence, while his brother carloman established himself in austrasia, alemannia and thuringia. the brothers were active in suppressing revolts led by the bavarians, aquitanians, saxons, and the alemanni in the early years of their reign. in 743, they ended the frankish interregnum by choosing childeric iii, who was to be the last merovingian monarch, as figurehead king of the franks.

1960: wilma rudolph was the first woman to win three olympic gold medals in track and field at one olympic games. she was an american sprinter from tennessee, she was acclaimed the fastest woman in the world. she was among the most visible black women in america and abroad. she became a role model for black and female athletes. she was also known as a civil rights and women right's pioneer.

1973: billy jean king is an american former number one tennis player. she represented the united states in the federal cup and the wightman cup. for three years she was the usa's captain in the federation cup. she defends gender equality and has been a pioneer for equal and social justice. at the age of 29, she won "the battle of sexes" tennis match against the 55 year old bobby riggs. she was also the founder of women's tennis association and the women's sports foundation.  

1983: tamara mckinney becomes the first american female skier to win the alpine world cup overall championship top. she was the only american to hold that title for a quarter century until 2008.

1992: manon rhéaume, from canada,   becomes the only woman to start in a national hockey league game in an exhibition game for the tampa bay lightning . she played on the canada women's national ice hockey team winning gold medals in 1992 and 1994. and silver medals in 1998. manon marked the first time a woman appeared in a regular season professional game with her first appearance against the salt lake golden eagles.

2013: serena williams won 78 of 82 matches at the age of 31. she is an american professional tennis player. she reached the number 1 ranking for the first time on july, 8, 2002. at the age of 16, she played against karsten braasch the battle of sexes. she was the highest paid female athlete in 2016. she is a unicef ambassador .