Step 1: Understanding Redressal Agencies. 
  
The Consumer Protection Act provides mechanisms for addressing consumer grievances. Redressal agencies have been established at various levels—National, State, and District forums. 
Step 2: Analyzing the options. 
  
- (A) National commission: Correct, the National Consumer Disputes Redressal Commission (NCDRC) is a redressal body at the national level. 
  
- (B) State commission: Correct, each state has a State Consumer Disputes Redressal Commission (SCDRC). 
  
- (C) District forum: Correct, District Forums handle consumer disputes at the district level. 
  
- (D) All of these: Correct, as all three are redressal agencies under the Consumer Protection Act. 
Step 3: Conclusion. 
  
The correct answer is (D) All of these, as the Consumer Protection Act provides for redressal agencies at all three levels: National, State, and District.