Which of the following is not a valid reason to appropriate retained earnings?

a$.$ A firm appropriates retained earnings to assure maintenance of a specific debt to equity ratio in accordance with a contractual debt covenant.

b$.$ A firm appropriates retained earnings for a probable and reasonably estimable lawsuit loss.

c$.$ A firm appropriates retained earnings for the cost of treasury stock in order to meet requirements of state law.

d$.$ A firm appropriates retained earnings to alert shareholders that a portion of retained earnings is unavailable for dividends.