We are using "the" article which is a specific article therefore the translation should यह साबुन किसका है?
यह साबुन किसका है? translates as: Whose soap is this? OR Whose is this soap?
By this reverse translation, I am trying to show the difference between definite article and demonstrative determiner.
I hope it will help :)
Can somebody help me i have confusion between whose and whom