Final answer:
A mechanic's lien is the correct answer as it represents a security interest in the title of real property, which is an encumbrance constituting a lien.
Step-by-step explanation:
The question revolves around the concept of encumbrances in real estate and which ones would constitute a lien on real property. An easement in gross is a right to use the property of another for particular purposes, like utilities or access, but it does not place a financial claim on the property. A covenant running with the land refers to conditions tied to the use of land that bind the current owner and all future owners, this also does not represent a lien. A mechanic's lien, on the other hand, is a security interest in the title of real property for the benefit of those who have supplied labor or materials that improve the property, which indeed constitutes a lien. Lastly, an adverse possession claim does not create a lien; rather, it can result in someone gaining ownership of land through prolonged and unauthorized occupation under certain conditions.