Due to emergency conditions, the army was asked to . . . . . . . .

The correct answer is C. stand by.

To stand by means to be ready to act or be used if needed. In this case, the army was asked to stand by because they were needed to help with an emergency.

The other options are not as accurate. To stand out means to be noticeable or different from others. To stand up means to rise to one’s feet, especially in order to show respect or to make a speech. To stand in means to take the place of someone else, especially temporarily.

In this case, the army was not asked to be noticeable or different from others, to rise to their feet, or to take the place of someone else. They were asked to be ready to act or be used if needed.