Explanation: Standar English is the dialect of English language that is used as the national norm in an English-speaking country, it is an English that respects spelling, grammar, pronunciation, and vocabulary. From the given sentences, the one that uses standard English is "They bought themselves a new car" because "themselves" is the correct form of the reflexive pronoun in plural (reflexive pronouns are: myself, yourself, himself, herself, itself and themselves), the word "theirselves" is incorrect.