In the Roman Numeral system, the symbol for zero does not exist.

To round off a number to the nearest hundred, we check the last two digits of the number.
If the last two digits form a number less than 50, we round down the number to the nearest lower multiple of 100. 
If the last two digits form 50 or more than 50, we round up the number to the nearest higher multiple of 100.
To round off 30982 to the nearest hundred, our options are either 30900 or 31000.
Since 82 is greater than 50, 30982 is rounded off to 31000.

If we look carefully, the pattern is:
4-2 = 2
7-4 = 3
11 -7 = 4
16 - 11 = 5
Thus, the differences between the consecutive members of the pattern themselves form a pattern - 2, 3, 4 and 5.
Thus, the next member of the pattern would be obtained by adding 6 to the previous member, 16.
16 + 6 = 22
Thus, the missing number is 22.

The largest 6 digit even number is 999998. Numbers that end with 0, 2, 4, 6 and 8 are called even numbers.
999998 > 999996.

The symbol for 500 in roman numerals is D.

The predecessor of a number is one less than the number.
The successor of a number is one more than the number.

The successor of 9879 is 9879 + 1 = 9880.
The predecessor of 2589 is 2589 - 1 = 2588.
The predecessor of 7899 is 7899 - 1 = 7998.
The successor of 1230 is 1230 + 1 = 1231.
Hence options B and D are correct.

When we round off a number to the nearest 100, we check the last two digits of the number. If the last two digits form a number less than 50, we round down the number to the nearest lower multiple of 100. If the last two digits form 50 or more than 50, we round up the number to the nearest higher multiple of 100.
Since the number 3255 has to be rounded off to the nearest hundred, our options are 3200 or 3300. 55 > 50.
Hence the length of River Ganga when rounded off to the nearest hundred will be 3300 km.