Order does not matter so use "n choose k" formula is used to find number of unique combinations.
c=n!/(k!(n-k)!) where n is total possible choices and k is number of selections.
c=4!/(2!(4-2)!)
c=4!/(2!2!)
c=24/(2*2)
c=24/4
c=6
So there are 6 different two topping options when there are four different toppings to choose from.
