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

The correct sentence on the above choices is letter D: The bird flew north of the beach.  

A sentence basically has a subject, the object of the sentence or doer of the action and predicate, the verb, the adjective, etc. Sentence structures could be simple (one independent clause), compound (two independent clause with coordinating conjunction), complex (a subordinate & independent clause) and compound-complex sentences (subordinate & two independent clause). These include clauses, conjunctions, coherence and balance and even to the number of words you use in your subject and predicate. The benefit of complex or compound sentences is that it could give you more explanation on the subject or topic of the sentence. This gives you a much more understanding on what the sentence is trying to portray or to message to give.              

A. The three girls might of broken the records.
B. We should of left early.
C. The boys surrendered, but they should of fought.

Each of these have incorrectly used the word "of'. They should have used "have" instead.

The only sentence that uses" of" correctly is D. The bird flew north of the beach.