In substitution type of questions, some particular objects are assigned code names. Then a question is asked that is to be answered in the code language.
If diamond is called gold, gold is called silver, silver is called ruby, ruby is called emerald, which is the cheapest jewel ?
We know that 'silver' is cheapest. But, as given 'silver' is called 'ruby'.
So, 'ruby' is the cheapest.
Hence, the answer is (d).