Step 1: Understanding financial planning characteristics. 
  
A sound financial plan must be simple to understand and execute, maintain liquidity to meet short-term obligations, and be economical in its approach to minimize unnecessary expenses while maximizing efficiency. 
Step 2: Analyzing the options. 
  
(A) Simplicity: This is true. A good financial plan should be simple to follow and implement. 
  
(B) Liquidity: This is true. A financial plan should ensure there is enough liquidity to handle daily operations and unexpected expenses. 
  
(C) Economicality: This is true. The plan should minimize costs and focus on efficiency. 
  
(D) All of these: Correct. All of these characteristics are essential for sound financial planning. 
Step 3: Conclusion. 
  
The correct answer is (D) All of these, as simplicity, liquidity, and economicality are all vital characteristics of good financial planning.