Final answer:
The number of exiles by June in 1873 was greater than 600. The only option greater than 600 provided in the multiple-choice answers is 900, therefore, the correct answer is (D) 900.
Step-by-step explanation:
The question involves a basic arithmetic calculation. We are given that the number of exiles at the beginning of 1873 was 400 and that by June the number had increased to more than 600. Although the exact number is not provided, we can deduce that it must be greater than 600 since that is the minimum number stated in the problem. Therefore, the only logical choice from the options given is (D) 900, as all other options are either equal to 600 or less.