I think it is possibly point b
a)
Check the picture below.
b)
volume wise, we know the smaller pyramid is 1/8 th of the whole pyramid, so the volume of the whole pyramid must be 8/8 th.
Now, if we take off 1/8 th of the volume of whole pyramid, what the whole pyramid is left with is 7/8 th of its total volume, and that 7/8 th is the truncated part, because the 1/8 we chopped off from it, is the volume of the tiny pyramid atop.
Now, what's the ratio of the tiny pyramid to the truncated bottom?

To find the decimal value, we must convert 1/2 and 3/4 into decimal.
1/2=0.5
3/4=0.75
Now we just have choose a random decimal in between the decimal.
Let's choose 0.6
Now we have to convert the decimal into a fraction. So we'll multiply the decimal by100.
0.6×100=60
Then we make the 60 into a fractions. 60/100
Then we'll simply the fraction. 60/100÷10=6/10÷2=3/5
There you go. You have your answer.
4$ would be the cost of the item