In ITIL 4, defining service requirements to ensure the service is fit for purpose involves specifying clear, measurable, and relevant criteria that align with the organization's needs and the service's intended use. Availability is a critical aspect of a service's fitness for purpose, as it directly impacts the service's utility.

Option A (Incorrect): Specifying data loss tolerance is important but doesn't provide a complete picture of service availability, which is a more comprehensive requirement.

Option B (Correct): This option is correct because it specifies a clear, measurable requirement that directly relates to the service's availability. Ensuring that the service is available 24/7 with 99.99% uptime is a crucial aspect of making sure the service is fit for purpose, as it ensures that the service will meet the expected operational requirements.

Option C (Incorrect): While retaining data for 10 years is important, it relates more to compliance and data management rather than the service being fit for purpose in terms of availability.

Option D (Incorrect): Combining data from different sources is a functional requirement, but it does not address the critical aspect of service availability or reliability, which are key to ensuring the service is fit for purpose.