Which of the following is a plausible lens mounting configuration?

A C-mount lens on a CS-mount housing with a 5-mm spacer is a plausible lens mounting configuration because C-mount lenses and CS-mount housings differ slightly in their back focal lengths. The CS-mount has a shorter back focal distance than the C-mount. By utilizing a 5-mm spacer, the configuration accommodates this difference, allowing the C-mount lens to achieve the appropriate focus and fit properly with the CS-mount housing. This allows for flexibility in selecting lenses for various camera setups.

When considering the other options, a CS-mount lens on a C-mount housing would not work appropriately without any spacers, due to the focal distance constraints. A 3-inch lens can’t be universally applied to any camera model due to variations in mounting systems and required focal lengths, making it impractical. Lastly, while it's common to use standard lenses in surveillance housing, the term "standard lens" lacks specificity, and not all lenses labeled as standard will fit or perform well in every surveillance setup. Hence, the rationale supports that Option A is the most viable configuration in this context.