The 'service desk' practice in ITIL 4 focuses on providing a single point of contact between the service provider and users. It is responsible for capturing demand for incident resolution and service requests, while also being responsible for general communication across the entire organization.

A. Including service desk communication in every value stream: While communication is a key component, service desk communication is not necessarily included ineveryvalue stream. The service desk is a component of user interaction, but the success factor here focuses more on how communication is handled rather than its integration in every part of the value chain​.

B. Ensuring that multichannel communication is used and improved wherever possible(Correct Answer): One of the key success factors for a service desk is to ensure that communication happens across multiple channels such as phone, email, chat, or web portal. This makes the service desk more accessible and flexible in how it interacts with users, and ITIL 4 stresses the importance of continually improving communication channels to enhance service desk effectiveness​​.

C. Enabling the effective integration of user communications into value streams: This option suggests that the focus is on how communications fit into value streams, but in the context of the service desk, success factors revolve more around communication quality and accessibility rather than integration into the value streams​​.

D. Overcoming the challenges associated with using web portals: While overcoming challenges with portals is important, it is not a broad success factor for the entire practice of the service desk. It is more specific to one communication method​.

Thus, the success factor that best aligns with the goals of the service desk practice in ITIL 4 is ensuring multichannel communication and its continual improvement.