Can someone plz

write how is it done plz

This is a confusing problem.  As I read it, it kept going
in and out of focus.  But I think I've got it now, and I also
think that NONE of the choices is correct.

We have to work with different values that 'a' and 'b' can be.
What are 'a' and 'b' ?   The question says that they're integers
that we pick so that  a·b  is always 36.  Can you think of another
way to say the same thing ? 

'a' and 'b' are a factor pair of 36 !  That will help us write down
all the possible pairs of 'a' and 'b'.  I'll do that in a second, but first ...
notice that the question is also going to talk about " a + b ",
so as I write down the factor pairs, I'll also write their sum next to
each one:

The factor pairs of 36 are:

1  and  36 . . . . . sum = 37
2  and  18 . . . . . sum = 20
3  and  12 . . . . . sum = 15
4  and  9 . . . . . . sum = 13
6  and  6 . . . . . . sum = 12 .

The question wants us to look through the list
for the biggest sum and the smallest sum, and
then figure out the difference between them.
That's easy !

The biggest sum is  37 .
The smallest sum is  12 .

The difference is    (37 - 12)  =  25 .

I don't see that among the choices.
But if I understand the question, then
that's my answer, and I'm sticking to it.

There are 15 rings on the laddey