How do you set a property to be read-only in an LWC?

To make a property read-only in a Lightning Web Component, the appropriate approach is to not use the @api decorator. The @api decorator in LWC is used to expose properties to the parent component, enabling them to be set from outside the component. If a property has the @api decorator, it becomes publicly accessible, allowing consumers of the component to modify it.

In contrast, if a property is simply defined without the @api decorator, it remains private to the component. This means that only the component itself can access and modify this property, thus making it effectively read-only from the perspective of any external consumer or parent component. This encapsulation of data helps to maintain the integrity and control of the component's internal state.

While making a property private helps to restrict direct access, the ultimate distinction for a read-only property in terms of accessibility is whether or not it is marked with the @api decorator. Therefore, the correct answer is that a property is set to be read-only by not using the @api decorator.