Traditionally, immigrants came to america from europe. this was no longer the case by the 1970s, largely because a. european nations refused to let their citizens leave. b. most european nations were becoming increasingly economically stable and democratic. c. europeans ceased to see america as a land of opportunity. d. america closed its borders to all european immigrants.

I would say you're answer is B. Most European countries at this time becoming democratic, so I would say this about right.
For A, I don't remember Europe ever putting a travel or moving ban on European citizens to America, there wouldn't be a reason to anyway at this time frame (1970's).
For C, it could be correct for many Europeans, but I wouldn't say this was the definitive answer. Many European's today still travel and move to the US for its opportunities. 
For D, I know America put immigration quotas on European immigrants back in the 1920's, but never completely closed their borders from them. I don't think this would apply at all. 

Hope this helps :)

I would say you're answer is B. Most European countries at this time becoming democratic, so I would say this about right.
For A, I don't remember Europe ever putting a travel or moving ban on European citizens to America, there wouldn't be a reason to anyway at this time frame (1970's).
For C, it could be correct for many Europeans, but I wouldn't say this was the definitive answer. Many European's today still travel and move to the US for its opportunities. 
For D, I know America put immigration quotas on European immigrants back in the 1920's, but never completely closed their borders from them. I don't think this would apply at all. 

Hope this helps :)