First, notice that the difference between each neighboring term is different, so it is not linear, so we can rule out A and B. Now, it's all about finding a pattern. Eventually, through guess and check, you will get 2 * f(n - 1) + 1.
The mistake is that for the first step instead of distributing 2 to 3p and 1 they added 8+2 first. your actually supposed to distribute and get, 8+6p+2, then combine like terms and get 10+6p