The 'Collaborate across boundaries' principle is essential for programmes that inherently involve multiple organizations and stakeholder groups. The most significant contributor to achieving this principle is establishing a governance framework with clear decision-making processes that function effectively across these organizational divides. This ensures that all parties can contribute to and agree upon the programme's direction, fostering a unified approach and preventing silos. Without effective cross-organizational decision-making, collaboration remains superficial, and the programme is at risk of fragmentation and conflicting priorities.

A. This describes an aspect of the 'Assurance' governance theme and the 'Manage risk and opportunity' principle, not the core of inter-organizational collaboration.

B. While information sharing is a vital part of collaboration, it is an enabler rather than the most fundamental contributor. Effective governance (as in D) is required to use that information for joint decision-making.

C. This statement directly relates to the 'Focus on outcomes and benefits' principle and the 'Design' theme, concerning the pace and planning of delivery, not the method of working together.

AXELOS. (2020). Managing Successful Programmes (MSP) 5th Edition. TSO (The Stationery Office).

Section 3.7, Collaborate across boundaries: This section emphasizes that "Effective collaboration requires a culture of trust and transparency, supported by governance arrangements that work across organizational boundaries." It highlights the need for "clear roles, responsibilities, and decision-making authorities" to facilitate this collaboration. This directly supports option D as the most critical element.

Section 3.6, Manage risk and opportunity: This section details the risk-based approach, aligning with option A but as a separate principle.

Section 3.2, Focus on outcomes and benefits: This principle is described in terms of delivering value and aligning with strategic objectives, which is the focus of option C.