Explanation:

- Willful destruction refers to the intentional act of causing damage or destruction.

- Option 1: Disaster typically refers to a sudden event causing great damage or loss, often due to natural or severe conditions, but it doesn't necessarily imply willful intent.

- Option 2: Combustion denotes the process of burning something. It is not related to destruction unless specified.

- Option 3: Calamity means a serious event causing distress or disaster, often unexpected and with significant consequences.

- Option 4: Sabotage involves intentionally damaging or destroying something, usually for disadvantage. It's a willful act of destruction.

Sabotage is the best one-word substitute for willful destruction.