The correct answer to the sentence is letter B. well
I haven't felt well since I accidentally drank the expired milk.
We do not use letter A: well because this denotes that the action has already been done. The ‘good’ here signifies that the action has already been done. In ‘well’, the action from the past up until to the present is still ongoing, that is, the person narrating still felt ill since the day she drank the expired milk.