Answer:
D. The larger multiple
Explanation:
When the number is halfway between two multiples of ten, its first(concerned) digit would be 5. For example, the number halfway between 10 and 100 is

The number halfway between 100 and 1000 is
and so on..
In order to round 550 to nearest multiple of 10, we have to look for the digit at hundreds place. Since this digit in 550 is 5(the concerned 5 has been made bold), 550 will be rounded to the higher multiple, i.e. 1000
Similarly, 55 will be rounded to 100.
From here we can conclude that a number halfway between two multiples of ten will be rounded to the larger multiple of 10.