While eating nutrient-dense food is indeed important for overall health and can contribute to maintaining a healthy weight, the specific guideline that directly addresses weight maintenance as per the Dietary Guidelines for Americans focuses on balancing calories eaten with activity level.
Nutrient-dense foods provide more nutrients relative to their calorie content, which supports good health and can help manage weight, but the principle of balancing calorie intake with physical activity is the primary recommendation for maintaining a healthy weight.
So, both are important, but the best answer specifically related to maintaining a healthy weight is still "Balance calories eaten with activity level."