Correct Answer: Negligent

Solution : The fourth option is the correct answer.

Vigilant means watchful, alert, and attentive, often to detect and respond to potential dangers or problems. On the other hand, negligent means failing to take proper care or showing a lack of attention and caution.

The meanings of the other options are as follows:

• Stagnant means not moving or flowing; lacking progress or vitality.
• Dubious means hesitant, doubtful, or uncertain.
• Robust means strong, healthy, and vigorous.

Therefore, the correct answer is negligent.