The correct answer is (a) 30 days of it’s filing.
A caveat is a formal notice that is filed with a court to prevent a particular action from being taken. In the context of land registration, a caveat is filed to prevent a property from being sold or transferred without the caveator’s consent.
A caveat is only valid for a certain period of time, after which it expires. The period of validity is usually 30 days, but it can be extended by the court. If a caveat expires, the action that it was intended to prevent can proceed.
The other options are incorrect because they are not the correct period of time for a caveat to remain in force.