Please answer this fast..........

Dear student
a) She would have missed adding baking powder in the baking process. In baking powder,  tartaric acid is present which  neutralizes the bitterness produced by baking soda. 

b)The cake tasted bitter due to baking soda (chemically sodium bicarbonate or sodium hydrogen carbonate). Sodium bicarbonate is basically a mild base, and we know that bases are bitter to taste. 

c)For a cake which is already baked, and bitter, nothing much can be done. She can just add some sugar syrup on top of it to neutralise the bitter taste and she should be alert in future, to avoid such mistakes.

Regards