A built-up refrigeration system is characterized by components that are?

The correct answer highlights that a built-up refrigeration system consists of components that are specifically selected and assembled by the installing contractor. This process allows for customization based on the particular needs of the installation environment, which can include factors like space limitations, specific refrigeration requirements, and efficiency goals.

Contractors choose various components such as compressors, condensers, evaporators, and control systems that are best suited to meet the unique requirements of a particular application. This flexibility in component selection and assembly is a key characteristic of built-up systems, enabling tailored solutions that may enhance performance compared to simplified, pre-manufactured units.

The other options imply different levels of assembly and design, such as pre-assembled units or only custom-designed elements, which do not reflect the nature of built-up systems that emphasize a contractor's role in selecting and assembling individual parts rather than relying solely on ready-made or exclusively custom solutions.