This number should be 10.
The tens digit the the 1st number on a two-digit number. Because the highest number you can go without creating a hundredths place is a 9, and the question says the ones-place (the last digit), this has to make this number 0. If it were to be 1 or anything higher, it would have to create a hundredths place if the rule of the question were to be followed.
Example:
If ones-place were 1, then following rule: 1+9 = 10
Your number would end up being 101 instead of a tenths place number.
I hope that made some sense somewhere!