In management and administration, a "plan" is a blueprint for future action. For a plan to be effective and successful in achieving its goals, it must possess several key characteristics.
Step 1: Analyze the options.
(A) Flexible: A plan must be adaptable to changing circumstances. Since the future is uncertain, a rigid plan can fail if the environment changes.
(B) Efficient: Efficiency refers to achieving the maximum possible output or objective with the least amount of friction or wasted effort.
(C) Economical: A plan must be cost-effective. The resources (time, money, labor) spent on executing the plan should be justified by the results achieved.
(D) All of these: A high-quality plan incorporates all these elements—flexibility to handle change, efficiency for performance, and economy for cost-effectiveness.
Step 2: Conclusion.
Since all the listed qualities are essential for a robust and successful strategy, the correct choice is "All of these."
Final Answer:
\[
\boxed{(D) \, \text{All of these}}
\]