The greatest challenge to managing an organization’s end-user devices is having an incomplete end-user device inventory. An end-user device inventory is a document that records and tracks all the devices that are owned, used, or managed by the organization’s end-users, such as laptops, tablets, smartphones, etc. An end-user device inventory helps to identify and classify the devices based on their type, model, location, owner, status, etc. An end-user device inventory also helps to monitor and control the devices, such as enforcing security policies, applying patches and updates, detecting and resolving issues, etc. Having an incomplete end-user device inventory could lead to a lack of visibility and accountability for the devices, which could increase the risk of data loss, theft, or compromise, as well as the cost and complexity of device management. The other options are not as challenging as having an incomplete end-user device inventory, although they may also pose some difficulties or limitations for the device management. Unsupported end-user applications, incompatible end-user devices, and multiple end-user device models are all factors that could affect the functionality and compatibility of the devices, but they do notnecessarily affect the visibility and accountability of the devices. References = Risk and Information Systems Control Study Manual, Chapter 3, Section 3.2.1, page 3-11.