What is the halftime of a mean-reverting process?

(a) It is the time taken for the price to revert to zero.

(b) It is the time taken for the price to revert half way to its long-term level from zero.

(c) It is the time taken for the price to revert half way to zero from its current level.

(d) It is the time taken for the price to revert half way to its long-term level from its current level.