Choose the sentence below in which of is used correctly.
a. we should of left early.
b. the three girls might of broken the records.
c. the boys surrendered, but they should of fought.
d. the bird flew north of the beach

The bird flew north of the beach is the answer.

It is that one because A should have been " We should have left early."
B should have been " The three girls might have broken the records."
C should have been " The boys surrendered, but they should have fought."
However if you put D in that context " The bird flew north have the beach."

In the context that A, B and C are in it doesn't make sense. 
Hope this helped. :)