A person obsessed with exclusively one idea or subject

Answer: Option C

One word substitute is Monomaniac.

Crazy:mad, especially as manifested in wild or aggressive behaviour.
Kleptomaniac: the inability to refrain from the urge for stealing items and is usually done for reasons other than personal use or financial gain.
Monomaniac: a person exhibiting an exaggerated or obsessive enthusiasm for or preoccupation with one thing.
Nymphomaniac: a woman with uncontrollable or excessive sexual desire.