Which of the following is part of the OPC specifications?

OPC Data Access is indeed a fundamental part of the OPC (OLE for Process Control) specifications. These specifications were developed to facilitate interoperability between different industrial software applications and devices in the automation sector. OPC Data Access specifically refers to the standards that allow real-time access to data from automation systems, enabling applications to read and write data efficiently and reliably. This standard covers aspects such as the communication of process data, including status information and measurements, thereby forming a critical component of the overall OPC framework.

The other options do not represent established parts of the OPC specifications. For instance, OPC Cloud Services and OPC Mobile Access may imply future or conceptual expansions of the OPC standards, but they do not exist as recognized lower layers or integrity levels. Similarly, OPC Secure Network does not correspond to an official OPC standard. The emphasis on Data Access reflects the core purpose of OPC, which is enhancing connectivity and communication in industrial environments.