Choose the right answer to make a complete compound sentence.
She was tired, ______________ she went to bed early.

The correct answer is: For

We are required to fill in the blank with a word that conveys the meaning "due to the fact that" (due to the fact that she was tired, she went to bed early). Option a means "up until now", option c means "in addition to" and option d means "in contrast to", so they are all eliminated. Thus the correct answer is option b.