The correct answer is B. tired.
If you work beyond your capacity, you will naturally feel tired. This is because your body is not able to keep up with the demands you are placing on it. You may also feel drowsy, but this is more likely to be due to lack of sleep than to working too hard.
Option A, drowsy, is a possible symptom of working beyond your capacity. However, it is more likely to be due to lack of sleep than to working too hard.
Option C, confident, is not a likely symptom of working beyond your capacity. In fact, you may feel more stressed and anxious if you are working too hard.
Option D, giddy, is also not a likely symptom of working beyond your capacity. This is more likely to be a symptom of excitement or happiness.