Which of the following complexities associated with AM would allow for an engineer to design moving features directly into a part build?

Functional complexity is the correct choice because it refers to the ability to design intricate and interrelated functionalities within a single part during the additive manufacturing process. This complexity allows engineers to create components that include moving features, such as hinges or internal mechanisms, which can be built as a single piece rather than requiring assembly of multiple parts.

Additive manufacturing excels in creating complex geometries and enables the incorporation of moving features that traditional manufacturing methods might not easily accommodate. The additive process allows for the layering of material in a precise manner, which supports the intricate design needed for functionality such as movement within the part itself.

The other options focus on different aspects of additive manufacturing. Material density concerns the weight and strength of the material used in the build, which is important but does not directly enable the integration of moving features. Surface texture deals with the finish and smoothness of the part's surface, affecting aesthetics and performance rather than design complexity. Cost efficiency is a critical factor in manufacturing decisions, but it doesn't influence the ability to design moving features into a single part. Each of these aspects plays a role in the AM process, but functional complexity most directly relates to the capacity to innovate and incorporate moving elements within the design.