Now that I think about it, John was a strange _______ he would never greet me if we found each other out of our work environment.
Which of the following options CORRECTLY completes the sentence above?

“Now that I think about it, John was a strange man; he would never greet me if we found each other out of our work environment.” is grammatically correct. All the other answer choices are run-on sentences.
Run-on sentences are when two or more complete sentences are written as one sentence. Run-on sentences can be fixed by adding a dash (-), a semicolon (;), or a colon (:) between the two sentences. You could also add a coordinating conjunction with a comma (, but) or make them two separate sentences with a period (.).
Any of these would be considered correct:
Now that I think about it, John was a strange man – he would never greet me if we found each other out of our work environment.
Now that I think about it, John was a strange man; he would never greet me if we found each other out of our work environment.
Now that I think about it, John was a strange man: he would never greet me if we found each other out of our work environment.
Now that I think about it, John was a strange man, because he would never greet me if we found each other out of our work environment.
Now that I think about it, John was a strange man. He would never greet me if we found each other out of our work environment.