If you made a stack of nickels 100 inches tall how many nickels how many nickels would you need

To answer this question, you'd need to know the approx. thickness of a nickel. Was this info not given to you in the problem statement?

Let's assume that the thickness of a nickel is 1.5 mm.  What fraction of an inch is that?

1.5 mm      1 inch         1 cm
* * = 0.06 inch
      1         2.54 cm       10 mm

How many nickels would it require to make a stack 100 inches tall?

Divide:

100 inches
= 1667 nickets (approximately)
 0.06 inch / 1 nickel

Note that this response is based upon my estimate that a nicket is 0.06 inch thick.  If you want a more accurate answer, you'll need to measure the thickness of a nickel or look it up somewhere.

Iam thinking the answer is a.) knew