What type of guard is used to protect a unit in a tactical environment?

In a tactical environment, the appropriate term for the type of guard used to protect a unit is typically an **exterior guard**. An exterior guard focuses on protecting the perimeter and keeping threats away from the main body of the force. Interior guards, on the other hand, tend to focus on protecting assets or personnel within a secured area.

In military terminology, a **main guard** could also refer to the primary force designated for overall protection, but it is less specific than "exterior guard." The term "right guard" is not commonly used in a tactical context in the same manner.

So, the best answer here is **exterior guard**.