Rule 8 targets a specific kind of closeness between an advocate and an organisation, closeness that comes from holding real decision-making power within it. Ranking the four bodies named in the options by how much operational control they actually carry over the organisation helps identify which one Rule 8 is aimed at.
Only membership of the Executive Committee places an advocate in a position of genuine managerial control over the organisation whose case he would be arguing, which is the precise conflict Rule 8 exists to prevent.
Hence, the correct answer is Executive Committee.