One of the difficulties when integrating multiple source systems is:

Integrating multiple source systems is challenging due to the need to identify valid links or equivalences between data elements across systems (e.g., matching customer IDs). The DAMA-DMBOK states: “A key challenge in data integration is determining equivalences between data elements from disparate sources, requiring mapping and transformation to ensure consistency” (DMBOK2, Chapter 13: Data Integration and Interoperability, p. 513).

Options A, C, D, and E are challenges but less central: documentation (A) is operational, modifying systems (C) is often impractical, universal rules (D) are difficult but secondary, and architecture timing (E) is project-related.