Here's a problem to solve: Monica grows vegetable plants in her garden. The plants have holes in their leaves and look unhealthy. Which task should Monica do first to solve this problem?

Among the 4 following options, which is the correct answer?

- A: Add fertilizer to the soil.
 
- B: Water the garden more often.
 
- C: Plant different vegetables in the garden.
 
- D: Observe the plants to identify the source of the damage.
 