Explanation
The only input to the task of defining assumptions and constraints is stakeholder concern. Stakeholder concern is any issue, expectation, need, or preference that affects or is affected by the project or the solution.
Stakeholder concern helps to identify the factors that may limit or influence the feasibility, desirability, or viability of the solution. The other options are not inputs to the task of defining assumptions and constraints, but outputs or inputs of other tasks. The business case, the prioritized requirements, and the requirements are all outputs or inputs of the enterprise analysis knowledge area, not the requirements analysis knowledge area.
References:
What can be considered the only input to the task of defining ...
Business Analysis Knowledge Areas Explained as per BABoK
Specifying and modeling requirements - The Functional BA