By the end of last month, they _____________(结婚)for 50 years.

have been married

had been married
中文的结婚50年可不能直译啊，had married的意思是俩人结个婚用了50年，结没结成也都不一定呢。marry就不是个持续性的动词。

had been married

had been married

had married

By the end of last month, they _____________(结婚)for 50 years.
"had married"
for 50 years是典型的用完成时，而last month是上个月了，所以应该用过去完成时。