Which of the options below best describes the organisational structure of each organisation?

• Span of control refers to the number of subordinates a manager can effectively supervise.
• A large span of control (many subordinates per manager) typically results in a flat organizational structure with fewer layers of management.
• A small span of control (few subordinates per manager) typically results in a tall organizational structure with many layers of management.

Manager A at Organisation XYZ has a larger span of control (a team of 50 subordinates). This indicates that Organisation XYZ displays a Flat structure.

Manager B at Organisation ABC has a smaller span of control (a team of 5 subordinates). This indicates that Organisation ABC displays a Tall Structure.

• a. Organisation XYZ displays a Flat structure. (Correct, based on Step 2)
• b. Organisation XYZ displays a Vertical structure. (Incorrect, Vertical is generally associated with a tall structure, not flat)
• c. Organisation ABC displays a Diagonal structure. (Incorrect, Diagonal is not a standard organizational structure type in this context)
• d. Organisation ABC displays a Tall Structure. (Correct, based on Step 3)
• e. Both organisations display a Horizontal Structure. (Incorrect, they have different spans of control, so their structures would differ)

The correct statements are 'a' and 'd'. Option C combines these two statements.

The final answer is $\boxed{\text{C. a, d}}$