I think that the better answer, just given the way that the question is worded, has to be B -- The Great Famine. I only base this on the wording of the question, though, because you really could argue that both of these are the right answer. Here's why:
The horrible famine that hit Ireland in the 1840s was called the Great Famine. This is the one that caused so many Irish to emigrate to the US.
But the Great Famine was caused by potato blight. So you can argue that potato blight is the right answer.
But if you look at the question, it seems to refer to the famine and not to the cause of the famine. It seems that it is asking for what the famine is called, which means B has to be right.