Which of the alternatives complete the dialog below correctly?

P.S.: Could you please explain your answer to me? Why didn't you choose another alternative? Please explain it to me using grammatical rules?

BOSS: Remember , John I am counting on you. Do not let me down.

JOHN: Don't worry, boss______.

A) I don't
B) I won't
C) I am not
D) I didn't before and now I don't