The correct option is C Monomaniac
'A person obsessed with exclusively one idea or subject' is Option C, 'Monomaniac'. Option A, 'Pyromaniac' is a compulsion to set things on fire. Option B, 'Kleptomaniac' is inability to refrain from the urge for stealing items. Option D, 'Nymphomaniac' is a woman with excessive sexual desire. Thus, options A, B and D are incorrect and the correct word for the given phrase is option C, Monomaniac.