Which of the following is a necessity for a successful service level agreement (SLA)?

Comprehensive Explanation:

For an SLA to be effective, ITIL stresses:

Clear, simple, and agreed language

Shared understanding between the provider and the customer

Terms that are meaningful to both parties

ITIL warns against:

Using technical metrics that only the provider understands

Carrying outdated SLAs forward

Avoiding user-experience metrics — these are valuable

Thus, Option A is the required necessity.