ICAO Annex 17
Chpater 3. Physical Characteristics
3.14 Isolated aircraft parking position

3.14.1 An isolated aircraft parking position shall be designated or the aerodrome control tower shall be advised of an area or areas suitable for the parking of an aircraft which is known or believed to be the subject of unlawful interference, or which for other reasons needs isolation from normal aerodrome activities.

3.14.2 Recommendation.— The isolated aircraft parking position should be located at the maximum distance practicable and in any case never less than 100 m from other parking positions, buildings or public areas, etc. Care should be taken to ensure that the position is not located over underground utilities such as gas and aviation fuel and, to the extent feasible, electrical or communication cables.

As for the options:

At an isolated aircraft parking position. ---> CORRECT. The purpose of isolated parking is protect passengers, crew, ground personnel, and facilities from potential danger (explosives, armed individuals, etc.). This position is a predetermined, secured area with easy access for emergency, law enforcement, and security services, and at a safe distance from terminals and other aircraft.

At the gate closest from the control tower. ---> INCORRECT. This would expose terminal areas and people to potential threats, exactly the opposite of what ICAO procedures require.

At the first available gate. ---> INCORRECT. This is unsafe because the situation involves a security risk. Aircraft under unlawful interference must not mix with normal airport operations or passenger gates.

On the taxiway closest to the rescue and firefighting service station. ---> INCORRECT. Although proximity to firefighting might seem helpful, this location is not secure or isolated and could block emergency routes or other aircraft