What type of easement allows Mary to drive over Sue's land to access a boat dock?

The correct type of easement that allows Mary to drive over Sue's land to access a boat dock is an easement in gross. This type of easement is specifically designed to benefit an individual or a legal entity rather than a specific piece of land. In this case, Mary is granted the right to cross Sue's property for the purpose of accessing the dock, and this right is not tied to any land that Mary owns but is rather a personal right granted to her.

Easements in gross can be useful for access situations, such as this one, where someone needs to traverse another's land to reach a utility, resource, or in this case, a boat dock. This classification distinguishes it from other easements that are typically associated with the land itself, such as those that would run with the land.

The distinction between easements in gross and other types of easements highlights the idea that the benefit of the easement is personal to an individual or entity rather than linked to a property, effectively making it a suitable fit for this scenario.