The weimar republic was unpopular because a.) it was harsh regime b.) people felt the constitution was not fair c.) it did not promote german business well enough d.) it signed the treaty of varsailles which lead to a serve economic downturn in germany

The treaty was very lenient, as the French demands for revenge were overruled by the British and Americans. The French wanted security, by weakening the German military. The fact that Germany remained as a single country, and not partitioned, is thanks to the Anglo-Americans. The British wanted a trading partner, not a weakened state on the verge of collapse.

You have missed the point completely about why the Weimar Republic was so unpopular. The Treaty of Versailles did not impose a war guilt clause on Germany. In fact, read it. It says nothing about war guilt nor does it say that Germany started the war. What you have done is repeat the German propaganda. What made the Germans so off at the treaty was because they believed it imposed guilt upon them, not that it did. As for reparations, they were not impossible to pay: Germany was - when you read the treaty and study the history of what followed - only required to pay 50 billion gold marks (Around 30 billion dollars). They didn't want to, that is the simple point. When forced, they paid around 20 billion gold marks - largely within the period of 1919-21. Another 30 billion spread out over decades - appeasing the Allies - would have been very possible. Instead, they ended up getting the deal changed to one of having to pay a larger sum over seven decades: again, very possible.

The simple answer, boils down to what ammianus said: they got the blame for the armistice and the peace treaty. The fact that they never accepted the peace treaty - until 1925 - fostered more hatred. By that point, still arguing over how reparations were destroying their country - another lie and propaganda perpetrated to undermine the entire peace treaty - fostered even more trouble. That coupled with the economic problems the Weimar Government created - hyperinflation earlier in the decade - or were unable to deal with - the Great Depression and an increase in unemployment - led to the popularity of far right groups stating they would rectify all the problems and get rid of the things destroying the country (the government, the peace treaty etc).

Iwould have to say c. the german americans.