Final answer:
The value of the digit 9 in the number 6,924,000, representing the population of Hong Kong, is 900,000, as it is in the hundred thousand's place.
Step-by-step explanation:
The question involves understanding place value in a large number, specifically the population of Hong Kong. The value of the digit 9 in the number 6,924,000 is in the hundred thousand's place. Therefore, the value of the 9 is 900,000, which corresponds to option a) 900,000. The value of the 9 in the number 6,924,000 is 900,000. In this number, the 9 is in the millions place. Therefore, it represents 900,000.