Answer:
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.)
Step-by-step explanation:
Within these options, the most effective way to combine sentences 5 and 6 is option A, because called the AAGPBL is modifying the noun league without introducing any passive sentence or independent clause, which would hinder fluency in an unnecessary way.