A is the best answer because the house belongs to the two parents, meaning you have to use an apostrophe to make "parents" possessive. The example in B would indicate that you were going to the house belonging to ONE parent. The example in C is flat-out wrong because "parents" with no apostrophe is plural and so "house" doesn't get attached to their ownership. And finally, D is not an example that would be correct in any instance. "Parents's" isn't a word I believe could be correctly used anywhere unless maybe it was someone's name and you were using the newer convention of possessives ending in s getting their own apostrophe-s.