He was not able to unearth his secret to the last.

Answer: Option B

Suppress : forcibly put an end to.
Discover : find unexpectedly or during a search.
Disclose : make (secret or new information) known.
Decode : convert (a coded message) into intelligible language.
Unearth : discover (something hidden, lost, or kept secret) by investigation or searching.

Synonym of Unearth is Discover