Step 1: Understanding the qualities of a good plan. 
  
A good plan should be flexible to adapt to changing circumstances and conditions. Rigid plans can become ineffective when the environment changes. Flexibility allows an organization to respond to new challenges and take advantage of emerging opportunities. While a good plan can be time-consuming and sometimes expensive to develop, flexibility is a critical quality to ensure that it remains relevant in dynamic situations. 
Step 2: Analyzing the options. 
  
(A) Expensive: Incorrect. While a good plan may require resources, it is not defined by its expense. 
  
(B) Time consuming: Incorrect. A good plan might take time, but it is not characterized by being time-consuming. 
  
(C) Flexible: Correct. A good plan must be flexible to adjust to unforeseen circumstances and changing conditions. 
  
(D) Rigid: Incorrect. A rigid plan is inflexible and may not be able to adapt to changes, making it less effective in dynamic environments. 
Step 3: Conclusion. 
  
A good plan is flexible, ensuring it can be adjusted to meet changing circumstances. Thus, the correct answer is (C).