The correct option is D Euthanasia
Option D 'Euthanasia' means the painless killing of a patient suffering from an incurable disease. Option A 'Suicide' means act of killing oneself. Option B 'Euphoria' means happiness. Option C 'Gallows' means is a wooden structure, used to hang criminals from, as a form of execution.So, the correct word that can be substituted for given words is option D 'Euthanasia'.