The correct option is A who
A relative pronoun functions as a pronoun as well as a conjunction. It refers or relates to a noun that's common between two sentences, and joins those sentences together. The noun that the relative pronoun refers to, is called the antecedent. In the given sentence, "Raj and Tina" is the compound subject, a compound subject is a combination of two subjects that are joined with a coordinating conjunction like 'and'. The subject performs an action "bought" against the object "a house". The antecedent is the subject here, as it's the focus of the sentence, the relative pronoun relates "Raj and Tina" to the rest of the sentence. In the case when the subject is the antecedent, the relative pronoun 'who' is used, as opposed to 'whom', which refers to an object antecedent. Hence, option A is correct and C is incorrect. 'That' and 'which' are generally used for things, and not people. When 'that' is used to refer to a person, the sentence begins with it, but here it doesn't, hence it's the wrong option. Raj and Tina are people, hence 'which' won't be used and options B and D are incorrect.