Proper grounding procedures for a PLC installation specify that:

A) all enclosures, CPU and I/O chassis, and power supplies be connected to a single low-impedance ground.
B) paint or nonconductive materials should be scraped away to provide good ground connections.
C) all ground connections should be made using star washers.
D) all of these.

All of these options are important in properly grounding a PLC installation. Connecting all enclosures, CPU and I/O chassis, and power supplies to a single low-impedance ground ensures consistency in grounding throughout the system. Scraping away paint or nonconductive materials provides a better connection point for grounding, and using star washers helps to ensure a strong and reliable connection.