A property owner who has the legal right to use a neighbor's land holds a:

The correct choice is B, an easement, because an easement is a legal right that allows one property owner to use a specific portion of a neighbor's land for a particular purpose. This right is often documented and can specify the nature, duration, and location of the use allowed. For example, a common type of easement is a utility easement, which allows a utility company to install and maintain power lines across a property.

Understanding easements is vital since they affect property rights and the use of real estate. The holder of an easement does not own the land but has rights to a specific use, which can include access or utilities, depending on the type of easement granted. This legal concept is essential for navigating property law and real estate transactions.

The other options relate to different real estate concepts: an estate in land refers to the ownership of land and the interest one holds, police power involves the authority of the government to regulate land use for the public good, and encroachment pertains to a situation where one property owner's physical structure intrudes upon a neighboring property without permission. Each term has distinct implications regarding property rights and land use.