Final answer:
To access b. Lync Online on mobile devices, the necessary step is likely the installation of the Lync Mobile App, as it enables mobile connectivity to Lync services.
Step-by-step explanation:
The question revolves around the accessibility of Lync Online on mobile devices. If Windows devices can access Lync Online but mobile devices cannot, the most likely action needed is b) Lync Mobile App installation. Mobile Device Management policies are more about the security and management of mobile devices rather than specific app use. Updating Lync Online DNS records typically isn't related to device-specific access issues, and while a Secure Sockets Layer certificate is important for secure communication, it wouldn't be the reason mobile devices can't access Lync Online if Windows devices can already access it securely. The Lync Mobile App is designed to provide mobile devices with the capability to connect to Lync services, enabling users to participate in communications while on the go.
Updating Lync Online DNS records or securing SSL certificates are not specifically required to enable access for mobile devices. These steps are typically related to configuring the overall Lync Online environment and ensuring secure communication between clients and servers, but they are not directly related to enabling mobile device access.