The correct answer is "ones".

Please Recall the Place Value Chart.
For Example: The number 123.45 can be represented in the place value chart as:

Solution:

We write the given number in the place value chart.

To identify the place of the digit 9 in the number 0.9, we look for the position of 9 in the place value chart.

So, in number 0.9, the digit 9 is in the ones place.