Final answer:
Availability is not a source of concern for public clouds as they are built for high availability with redundant systems, contrary to privacy, confidentiality, and efficiency that might raise concerns.
Step-by-step explanation:
The aspect of systems reliability that is not a source of concern with regards to a public cloud is d. availability. Public cloud platforms are designed to provide high availability through multiple redundant systems and data centers. This means that they are typically better able to sustain operations and remain accessible even in the event of system failures or other issues, compared to private systems where the infrastructure is often more limited. Therefore, availability is generally seen as a strength of public clouds rather than a concern.