The correct answer is C. Two.
The hooter is a device that is used to sound an alarm. It is typically used to warn people of danger. In the case of an accident on a goods train at outside from ART HQ station, two hooters shall be blown. This is to alert people in the area of the danger and to help prevent further accidents.
Option A is incorrect because four hooters are not required to be blown in case of an accident on a goods train at outside from ART HQ station. Option B is incorrect because three hooters are not required to be blown in case of an accident on a goods train at outside from ART HQ station. Option D is incorrect because any one hooter is not sufficient to alert people in the area of the danger and to help prevent further accidents.