The committee set about making provisional arrangements for the annual conference.

Answer: Option C

Provisional : arranged or existing for the present, possibly to be changed later.
Unconditional : not subject to any conditions.
Abiding : (of a feeling or memory) lasting a long time; enduring.
Permanent : lasting or intended to last or remain unchanged indefinitely.
Lasting : enduring or able to endure over a long period of time.

Antonym of Provisional is Permanent