The quality criteria that must be satisfied when the BA is defining requirements relationships is necessary. This means that the BA should ensure that each requirement has a clear and valid reason or purpose for being included in the solution scope. A necessary requirement is one that supports a business goal or objective, addresses a problem or opportunity, or satisfies a stakeholder need or expectation.

The other options are incorrect because:

Option A is incorrect because priority is not a quality criteria that must be satisfied when the BA is defining requirements relationships, but a factor that the BA should consider when prioritizing requirements. Priority is the relative importance or urgency of a requirement based on its value, feasibility, risk, or dependency.

Option B is incorrect because complexity is not a quality criteria that must be satisfied when the BA is defining requirements relationships, but a factor that the BA should consider when analyzing requirements. Complexity is the degree of difficulty or uncertainty involved in implementing or maintaining a requirement based on its scope, scale, or interdependencies.

Option D is incorrect because source is not a quality criteria that must be satisfied when the BA is defining requirements relationships, but an attribute that the BA should document when managing requirements. Source is the origin or reference of a requirement such as a stakeholder, document, regulation, standard, etc.

References:

BABOK Guide v3, section 6.2: Requirements Analysis and Design Definition

CCBA Exam Questions, question 31