Final answer:
The object of the preposition in the sentence is the 'civil war,' which follows the preposition 'during.' The correct answer is option 3.
Step-by-step explanation:
To identify the object of the preposition in the sentence provided, we first need to locate the prepositions. The prepositions in the sentence "There was New Yorker A. G. Mills who played baseball during the civil war and also was once the national baseball league's president." are 'during' and 'of.' The object of the preposition 'during' is 'civil war,' and the object of the preposition 'of' is implicitly 'baseball league' (in 'the national baseball league's president'). Hence, the objects of the prepositions are 'civil war' for 'during' and 'baseball league' for 'of.' Since 'baseball league' is not an option in the given answers and the question likely refers to the first prepositional object, the correct answer is option 3, 'Civil war.'