What caused the great waves of immigration from Europe to the United States in the mid-1840's?

a.

The yearning for liberty

b.

the monarchy of Europe ordering people to leave

c.

the influence of Karl Marx

d.

the failure of Europe's crops

A is the best option. I feel none of these options are clearly detailed but, if you have to pick one it would be B. This is because many people came seeking religious freedom. D could also be correct as at the time many of Europe's crops weren't making as much as they usually did.