Two soldiers were involved in an animated discussion regarding the correct way to set up a perimeter defense. Animated most nearly means:
Explanation
An animated conversation is also a lively conversation. Therefore, choice "1" is the correct answer here. Context doesn't help you much in this one; you have to know that "animated" can mean lively or excited. Since you know what a "perimeter defense" is (from watching movies, perhaps), you may eliminate "humanitarian."