A(n) facility is one in which security is based on staff involvement, where doors are often not locked, and there is no perimeter fencing or walls.

An open facility is one in which security is based on staff involvement, where doors are often not locked, and there is no perimeter fencing or walls.

Explanation:

As it is stated, there are some open facilities, which rely on no other security measure than staff involvement, such as security guards who control the area. An example of open facility can be a public building which is available at all times during the day such as a library, a museum, etc.

a is the final ! : )