Opulent: this word means that something is luxurious, expensive, and ornately decorated, which we can assume a house with marble columns is.
The other words mean the following:
fortuitous: means lucky, happening by fortune (she could be lucky to see that house, but you don't necessarily dream of a 'lucky' house)
cantankerous: grouchy, crabby (this does not fit, you would not want a cranky house)
traverse: walk through/across (this is the wrong part of speech to fit in the sentence, plus its meaning could apply to any place you walk through)
