Answer: d-He started a new league called the All-American Girls Professional Baseball League (AAGPBL).
Explanation: In the given excerpt we can see the description of how the professional baseball experienced a shortage of good players because many of them volunteered to serve in the World War II, and how this caused the foundation of the All-American Girls Professional Baseball League. The most effective way to combine sentence 5 and 6 in order to avoid repetition is "He started a new league called the All-American Girls Professional Baseball League (AAGPBL)."
