The most effective way to combine sentences 5 and 6 is A. He started a new league called the All-American Girls Professional Baseball League (AAGPBL.)
Explanation:
Within these options, the most effective way to combine sentences 5 and 6 is option A, because<em> called the AAGPBL</em> is modifying the noun <em>league </em>without introducing any passive sentence or independent clause, which would hinder fluency in an unnecessary way.
