Which type of door can be forced either by simply breaking a window, if available, or by using a Halligan tool to force the door away from the frame?

The inward opening door is the correct answer because it is designed to open towards the inside of a structure. When attempting to gain entry, one effective method is to use a Halligan tool to pry the door away from the frame at the hinge side or to apply pressure on the door's lock mechanism. If breaking a window adjacent to the door, which provides access to the interior, is an option, it is also feasible to reach in and unlock or manipulate the door from the inside.

Inward opening doors typically present vulnerabilities that allow for forced entry, making them a target in emergency situations where prompt access is necessary. When forced, these doors may yield more readily compared to other types, especially when the locking mechanism is compromised.

The other choices involve mechanisms or designs that may require different methods for forcible entry, making them less effective in this specific scenario.