His knowledge of the subject is quite extensive.

Answer: Option C

Extensive : covering or affecting a large area.
Ordinary : with no special or distinctive features; normal.
Little : small in size, amount, or degree (often used to convey an appealing diminutiveness or express an affectionate or condescending attitude).
Limited : restricted in size, amount, or extent; few, small, or short.
Immense : extremely large or great, especially in scale or degree.

Antonym of Extensive is Limited