The correct option is D Somnambulism
'Walking in sleep' is called Option B, Somnambulism. Option A, 'Ventriloquism' is the ability of speaking without moving lips so that it seems voice is coming from elsewhere. Option C, 'Obsession' is unable to stop thinking about something or someone. Option D, 'Hallucination' is illusion or fantasy. Thus, options A, C and D are incorrect and the correct word for the given phrase is Option B, Somnambulism.