The most secure and practical solution is to create a separate, isolated network (SSID) for the legacy device. Modern routers often support guest networks or multiple SSIDs, which can be configured with older security protocols like WPA without downgrading the security of the primary network (which should use WPA2 or WPA3). This practice, known as network segmentation, isolates the less-secure device, preventing it from being a potential entry point for an attacker to access more sensitive devices on the main network.

B. Downgrading the entire network's security to WPA, even temporarily, is a significant security risk and is highly impractical.

C. Using a wired connection does not meet the user's stated requirement for a "Wi-Fi connection" that must use WPA.

D. Adding an older router can create network issues like double NAT and introduces another unmanaged, potentially insecure device to the network.

1. Carnegie Mellon University, Software Engineering Institute. (2021). Best Practices for Home Network Security. Retrieved from https://insights.sei.cmu.edu/blog/best-practices-for-home-network-security/. See the section "Segment Your Network" which recommends using guest networks for less-trusted devices.

2. University of California, Berkeley, Information Security Office. (n.d.). Minimum Security Standards for Networked Devices. Retrieved from https://security.berkeley.edu/resources/minimum-security-standards-networked-devices. Standard 5.1 discusses network isolation for devices that cannot meet higher security requirements.