The given number is 312.501. It is required to round to the nearest whole number.
To round the decimal number, check if the number in the tenth place is less than 5 or greater or equal to 5.
If the number is less than 5, simply write the number before the decimal point without any alteration to round up.
If the number is greater or equal to 5, then add one to the number in the unit place, then write out the new number before the decimal point to round up.
Notice that the number in the tenth place of the given number, 312.501 is 5, which is greater or equal to 5. This falls in the second condition above.
Hence, add one to the digit in the units place, (2+1) to get the number:
The required answer is 313 (option D).