Unions requiring gaskets or packings shall not be used in _____ lines.

The correct response highlights that unions requiring gaskets or packings should not be used in oil lines. This is because the properties of oil can lead to deterioration or degradation of the gasket or packing material, potentially resulting in leaks or failures at the joint. Gaskets and packings are generally less reliable in systems that carry oil due to the way that oil can chemically interact with various materials.

In general, the most critical considerations when working with unions and their applications pertain to the compatibility of the materials used in gasket and packing with the fluid being conveyed. Different fluids can have different effects on these materials, influencing their effectiveness and longevity. While unions can be effectively used in water, gas, or air lines, the specific challenges posed by oil make it essential to select alternative connection methods or materials that can better handle the dynamic conditions present in oil lines.