What is double interval?

The correct answer, which identifies double interval, refers to the distance between soldiers when their arms are raised shoulder high. In military formations, a double interval is specifically defined as a formation where there is more space between individuals in a rank, allowing for such arm movements without the risk of contact. This formation provides adequate spacing to ensure that soldiers can maintain their personal space while also facilitating movement and ensuring order within the ranks.

Understanding the context of the other choices illuminates why they do not represent double interval. The space between soldiers when standing at ease generally refers to a different spacing standard, and the distance measured while marching relates to movement rather than the specific measure of spacing at rest. Additionally, the space between ranks in a formation typically addresses the vertical distance from one row of soldiers to the next, rather than the lateral distance between individuals within the same rank, which is the distinctive aspect of double interval.