What additional marker is used alongside a small line to designate a duct access door?

The additional marker used alongside a small line to designate a duct access door is commonly represented by the abbreviation "AD." This designation is part of standard architectural and mechanical drafting practices, where specific abbreviations are used to clearly communicate the purpose and function of various elements in a building plan.

In this context, "AD" specifically stands for "Access Door," effectively indicating to contractors and builders that an access door is to be installed at that location for maintenance or inspection of ducts, which is essential for ensuring proper air flow and system functionality. This clear labeling helps avoid confusion during the construction process and ensures that all parties involved understand the requirements of the project related to ductwork accessibility.

Other options may represent different components or features in construction plans, but "AD" is the standard notation for duct access doors, making it the correct answer in this case.