Answer:

to the nearest one.
Explanation:
To round a number to the nearest one is the same as rounding to the nearest unit.
We want to round 51.8 to the nearest one.
Since the number after unit digit is 8 , we round up. That means we need to add 1 to the number in the unit digit and cut off the remaining part to the right of the unit digit.
Therefore

to the nearest one.