I believe the answer is A. passage B
If you the other sentences you notice that the structure is similar. To answer your question of what has the best variety, you want to see sentences that aren’t the same length, sentences that don’t start with the same word, and different vocabulary. Those aren’t the only things you can look for; however, that is a good place to start.
You know passage c and d are not the correct answer because they start with the same word each time.
Passage a is a little trickier because as you can tell, they start with different words. However, they are the same type of words—transition words.
Therefore leaving you will b as the correct passage with a variety of structure and length.
I hope that helps :)