Think of the decimal model. which number has 10 more tenths than 0.31?
a.1.31
b.0.41
c.0.32
d.0.21

The answer you are looking for is a. 10 tenths is equal to one whole number.

The answer is A.) 1.31 

When you add 10 more ones to the tenths place (which is right after the decimal, and to the right.) 

Your question turns into this. 
0.31 + 10 + 10 + 10 + 10 + 10 + 10 + 10 + 10 + 10 + 10 = 1.31

So the answer checks out. 

Good Luck! :)