Which of the following is permissible for low-pressure systems regarding fittings?

For low-pressure systems, the use of mechanical grooved couplings that are specifically listed for carbon dioxide (CO2) is allowed due to safety and compatibility considerations. These couplings are designed to effectively handle the operating conditions of low-pressure systems while ensuring leak-proof connections and structural integrity under various circumstances.

The listing for CO2 ensures that these components meet certain safety and performance standards, making them suitable for the specific requirements of low-pressure applications. This attention to specifications reduces risks associated with pressure fluctuations and the unique properties of CO2 as a refrigerant.

Other options do not align with the requirements and standards necessary for low-pressure systems. Manually fabricated fittings may not offer the same level of reliability or safety as those that are listed, and welded configurations can introduce risks if not executed correctly. Moreover, using any type of fitting without proper listing could compromise the system's performance and violate code requirements. Thus, the preference for listed mechanical grooved couplings is clearly based on ensuring safe and efficient operation within low-pressure contexts.