Conditional Formatting or If Then

Hello,

First time Excel Discussion user here! I'm trying to figure out which function is the best option to use and easiest to set up. The scenario is this.

I have a drop down menu in column 1 with a number of values to select from (A-D). Related to this is a second column with another set of values (yes, no, maybe, N/A). The difference is that when you select A in the first column, only yes and no can be selected.

What is the best way to set this up, so that if the user clicks a value in column 1, only those values that are applicable will appear in the drop down menu in column 2?

Thanks for your help.

Rochelle.