To let the grass grow under one’s feet means :

The phrase "to let the grass grow under one's feet" is an idiom that suggests someone is being idle, inactive, or neglectful. It indicates a failure to take timely action or a lack of initiative, allowing opportunities to pass by or problems to worsen. This inactivity might give others a chance to take advantage or lead to missed chances. It often implies a sense of laziness, procrastination, or lack of urgency in getting things done.thats why Option C is correct.