The correct answer is A. Non Tangible.
A non-tangible authority to proceed is a verbal or hand signal given by the train dispatcher to the train crew. It is used at stations without starter signals, where the train crew cannot see the next signal. The train dispatcher must be sure that the track is clear before giving a non-tangible authority to proceed.
A tangible authority to proceed is a physical device, such as a token or key, that is given to the train crew by the train dispatcher. It is used at stations with starter signals, where the train crew can see the next signal. The train dispatcher must be sure that the track is clear before giving a tangible authority to proceed.
Option C is incorrect because the train dispatcher must use a non-tangible authority to proceed at a station without starter signals.
Option D is incorrect because there is a correct answer to the question.