Answer:
a.
f(1)=6; f(n)=6+d(n-1), n>0
Explanation:
We are given that
First layer has squares, a=6
Second layer has squares, a2=12
We have to find an arithmetic explicit formula to determine the number of squares in each layer.

nth term of an A.P

Substitute the value of a
Now, we get

f(1)=a=6

Hence, option a is correct.
a.
f(1)=6; f(n)=6+d(n-1), n>0