The given number is 515.
It is asked to round off the number to nearest hundred.
This means that the resultant number will have zeroes in its unit and tens place.
Now consider that the tenth digit of the given number is 1, which less quite less than 5, so the digit 1 will be dropped and replaced by zero so that the place value of 5 remains entact.
Thus, the resulting number will be 500.
Alternative Explanation:
Just observe that the near hundred numbers to 515 are 500 and 600.
But 515 lies nearer to 500, so rounding off 515 to nearest hundred will yield 500.