Final answer:
Personal fall arrest snap-hooks must be locking and should be self-locking to ensure safety at heights.
Step-by-step explanation:
Personal fall arrest snap-hooks must be locking and be: 1) Self-locking, 2) Non-locking, 3) Semi-locking, or 4) Cannot be determined.
The correct answer is 1) Self-locking.
A self-locking snap-hook is designed to automatically lock closed and prevent accidental disengagement. This is crucial in ensuring the safety of individuals who work at heights. Self-locking snap-hooks provide an additional level of security and reduce the risk of falls or accidents.