Addition Property of Equality is basically a complex way of saying add the same number to both sides. In this problem we are trying to isolate x for it to be by itself so we must deal with -6. Since we have -6 on one side we must use the "Addition Property of Equality" to add 6 to both sides which would cancel out the -6 and add 6 to 14.
Therefore, the option that best fits the description on what number we would have to use to get added in the equation would be option D, 6.