Correct Answer:
B. but
The correct conjunction to fill the blank is 'but'. 'But' is used to introduce a contrast or an exception. In this sentence, 'large' describes a positive or neutral attribute, while 'quite old-fashioned' introduces a contrasting or slightly negative attribute about the house. 'And' (A) would imply addition, 'so' (C) would imply a result, and 'because' (D) would imply a reason, none of which fit the intended meaning of presenting a contrasting characteristic.