The correct answer is (A).
To mobilize means to prepare and organize (troops or other resources) for war or other conflict.
To summon means to call someone to come to a
particular place, especially in order to give them an order or to answer a question.To prepare means to make or get ready for something that is going to happen.
To arouse means to cause someone to feel a strong emotion, especially anger or excitement.
In this case, the troops were mobilized to take control of the tensed situation. This means that they were prepared and organized for war or other conflict. They were not summoned, as this would imply that they were called to come to a particular place. They were also not prepared, as this would imply that they were getting ready for something that was going to happen. Finally, they were not aroused, as this would imply that they were feeling a strong emotion, such as anger or excitement.