To turn over a new leaf A. To change completely one’s course of action B. To shift attention to new problems after having studied the old ones thoroughly C. To cover up one’s fault by wearing new marks D. To change the old habits and adopt new ones

The correct answer is A. To change completely one’s course of action.

To turn over a new leaf means to make a fresh start, to change one’s ways, or to begin a new phase in one’s life. It is often used in the context of making a change for the better, such as after making a mistake or going through a difficult

Option B is incorrect because it does not mean to change one’s course of action. It means to shift attention to new problems after having studied the old ones thoroughly.

Option C is incorrect because it does not mean to change one’s course of action. It means to cover up one’s fault by wearing new marks.

Option D is incorrect because it does not mean to change one’s course of action. It means to change the old habits and adopt new ones.