The correct option is A which I showed you
Option A is correct as the relative pronoun in the adjective clause of accusative/objective case functions as an object and is followed by a subject. Here, the relative pronoun 'which' takes the relative pronoun 'I' as its object and follows with the subject of the sentence 'showed you'.
Option B is incorrect as 'I showed you which' does not need 'which'. It would be absolutely fine as 'I showed you yesterday'.
Option C is incorrect as the use of the pronoun 'it' to refer to the house is incorrect as the relative pronoun 'which' already refers to it.
Option D is incorrect as 'I showed it' is incorrect grammatically.
Thus, option A is correct.