The correct answer is D. Only III is implicit.
The statement “Items in big showrooms in main markets are costlier than similar items in small shops” does not provide enough information to support any of the other assumptions. For example, it is possible that the items in small shops are just as reliable as the items in big showrooms, but that the small shops simply cannot afford to charge as much. It is also possible that the persons managing big showrooms are not cheats, but that they simply have to charge more in order to cover the costs of maintaining their large stores. Therefore, the only assumption that is implicit in the statement is that the maintenance of big showrooms is an expensive affair in itself; hence the prices are a little higher.