Explanation:

- Alert means being quick to notice or respond to danger or changes.

- Idle means not active or in use, often implying a lack of activity or movement.

- Careless suggests a lack of attention or thought, typically leading to mistakes or accidents.

- Sluggish refers to moving slowly or lacking energy.

- Lazy means unwilling to work or use energy, often being inactive or sluggish.

The nearest antonym for "alert" is Option 3: sluggish. Being alert is about being quick and responsive, while being sluggish means slow and unresponsive.