Step 1: Understanding Sensex. 
  
The Sensex is an index that tracks the performance of 30 major companies listed on the Bombay Stock Exchange (BSE). 
Step 2: Analyzing the options. 
  
(A) 30: Correct. The Sensex is based on the performance of 30 major companies listed on the BSE. 
  
(B) 50: Incorrect. The Sensex includes only 30 companies, not 50. 
  
(C) 75: Incorrect. The Sensex consists of only 30 companies. 
  
(D) 100: Incorrect. The Sensex includes 30 companies, not 100. 
Step 3: Conclusion. 
  
The correct answer is (A) 30, as the Sensex tracks the performance of 30 major companies listed on the BSE.