Explanation:

- Excruciating: Means intensely painful or severe. Often used to describe extreme discomfort or agony.

- Option 1: Facile: Means appearing neat and comprehensive only by ignoring the true complexities; superficial. Also used for something easily achieved.

- Option 2: Enticing: Means attractive or tempting; alluring.

- Option 3: Alluring: Similar to enticing, it means powerfully attractive or charming.

- Option 4: Piercing: Refers to something sharp or intense, often in terms of sound or sensation.

While “piercing” can imply intensity, it is synonymous with sharpness rather than pain. The correct synonym for "excruciating" is not explicitly listed but option 4 aligns more closely with the nature of excruciating, though not perfectly.