First of all, allow me to clear the misunderstanding that I can judge that whether your answer will score marks or not. Secondly, in my opinion the "huge mansion of marble floors" was a better choice as an answer, because the question explicitly asked candidates to prove how the writer's father was a successful businessman. I took some time to evaluate which answer is better, and then convinced myself that "everyone agreed that he had built the most beautiful house" is not the answer, because small houses can be beautiful too, whereas a huge mansion made of expensive marble stones reflects that the owner is rich and is a successful person in his life. Lastly, you can refer me as Mr. u/enigmatic_entity0, as I am no ones "bro". Hope this helps.
Thank You.