quieren would be the right answer where I speak Spanish but since all Spanish is considered "slang" other than the one they use in the class, this could be wrong, but that word is the word that goes with it the most and makes perfect sense.
also querríamos means everyone but in the sentence it just says his or hers cousins.
quierras wouldn't go with the sentence and doesn't make sense.
querria usually means one but in this sentence many of them wanted so that would be wrong and the last one would be right
