The correct option is D Unhealthy
The word 'nourishing' refers to a food containing substance essential for growth, health and good condition.
Option A, unhealthy refers to something harmful for health.
Option B, poor refers to being deficient or lacking in something.
Option C, undercooked refers to something cooked insufficiently.
Option D, heavy refers to something more than the usual size, amount or intensity.
Thus, according to the meaning, the word 'unhealthy' is the opposite of the word 'nourishing'.