Final answer:
By subtracting 2,000 from 644,353, the number that is two thousand less than 644,353 is found to be 642,353.
Step-by-step explanation:
The student asked: What number is two thousand less than 644,353? To answer this question, we subtract 2,000 from 644,353. The calculation is as follows:
644,353 - 2,000 = 642,353
Therefore, the correct answer is b) 642,353.