Let's break down the possibilities:
1. Size 7 ring in 18-karat gold - Since there are 4 different types of stones that can be set in each type of metal, the probability of finding a size 7 ring in 18-karat gold with any type of stone is 1/4.
2. Size 7 ring in platinum - Again, there are 4 different types of stones that can be set in platinum, so the probability of finding a size 7 ring in platinum with any type of stone is 1/4.
Since we are looking for the probability of finding a size 7 ring in either 18-karat gold or platinum, we need to add the probabilities from step 1 and step 2:
1/4 + 1/4 = 2/4 = 1/2 = 50%
Therefore, the probability of finding a size 7 ring in 18-karat gold or platinum is 50%.
Since none of the provided answer options match exactly, the closest answer is 50% or B. 16.6%.