For excel:
you plan to use the fill down feature on a formula and you need to keep a cell reference the same. which one of the following formats will allow you to keep the same cell reference?

I'm not sure what the 'following formats' are, but simply putting a $ in front of the letter will lock the column reference, and putting a $ in front of the number will lock the row reference. Putting a $ in front of both will allow you to keep the cell reference the same. For example:
A1 should be A$1$ in an equation to keep the reference as A1 for all 'filled down' equations.