A software design pattern often used to restrict access to an object is:

Explanation

The answer is proxy. A proxy provides a surrogate or placeholder that stands in for another object and controls access to it, which is exactly what a protection proxy, a virtual proxy or a remote proxy does. The adapter pattern converts one interface into another so that incompatible classes can work together, and the decorator pattern attaches extra responsibilities to an object at run time without changing its interface, so neither is about restricting access. Delegation is a general technique of forwarding a request to a helper object, not a GoF pattern for access control.