Lieutenant is not a part of the main clause, and it is an address to a person that is inserted into the sentence. It must be therefore separated by comas on both sides: the correct answer is a.
The correct answer is: Option A. Adopt the horses.
Given the context in the excerpt stating the individuals that would be "taking possession of the new horses" are official U.S. Marines, it can be inferred that this practice will take place within legal bounds, and therefore "stealing the horses" is out of context in this case. The intent of this activity is to use the horses as part of the Marines' activities, and as such, it is clear the Marines are planning to "adopt" the horses, not "borrow" them.
Answer:
depends on how you use the word though
