Final answer:
In the given options, a lean-to scaffold is not legal because it relies on support from structures that are not designed to handle additional loads, which presents a safety risk. Legal scaffolds must comply with occupational safety regulations such as those from OSHA, ensuring they are safe and capable of handling the required loads with necessary protective measures.
Step-by-step explanation:
The question regarding the legality of various types of scaffolds relates to regulations and safety standards typically associated with construction practices and workplace safety. In the options provided - horse, lean-to, stick, and metal - the 'lean-to' scaffold is generally not considered legal. A lean-to scaffold is a structure that is supported, at least in part, by another structure (usually a building), which is not designed to support additional loads, making it inherently unsafe and non-compliant with scaffolding safety regulations. In contrast, 'horse', 'stick', and 'metal' scaffolds, when constructed and used according to safety guidelines and standards, can be legal.
Legal scaffold constructions are typically regulated by occupational health and safety organizations, such as OSHA in the United States, which set forth requirements for design, construction, and use to ensure the safety of users and those nearby. For example, they require that scaffolds are able to support at least four times the maximum intended load and must be designed by a qualified person. The safety regulations also dictate the proper use of guardrails, toe boards, and other protective measures.