The correct answer is: The library is located behind the football stadium.
The verb “locate” means to find the position of something. In this sentence, the library is the subject and the football stadium is the object. The verb “locate” is in the present tense, which indicates that the library is currently located behind the football stadium.
The other options are incorrect because they do not agree with the tense of the sentence. The verb “has been located” is in the past perfect tense, which indicates that the library was located behind the football stadium at some point in the past. The verb “will be locating” is in the future tense, which indicates that the library will be located behind the football stadium at some point in the future.