What we call percentage is basically this:
We have a number, let's call it 'a' ( a can be 1,2,100,423432, whatever you want!)
Then we divide a by 100 and called the result 1%, in other words:

This is basically what a percentage is
So, in our problem, a=42in, as this is the final Kayla's height:
Then, we want to know what percentage of 42in is equal to 42in-36in=6in (change in Kayla's height)

So, the difference in percentage is: 14.3% which is the option C