The correct answer is: under control.
The word “under” is used to indicate that something is being controlled or regulated. In this case, the situation is being controlled or regulated, so the answer is “under control.”
The other options are not as accurate. The word “in” is used to indicate that something is located or contained within something else. In this case, the situation is not located or contained within something else, so the answer is not “in control.” The word “about” is used to indicate that something is concerned with or related to something else. In this case, the situation is not concerned with or related to something else, so the answer is not “about control.” The word “of” is used to indicate that something is a part of or belongs to something else. In this case, the situation is not a part of or belongs to something else, so the answer is not “of control.”