Final answer:
The correct possessive pronoun to complete the sentence is 'themselves'. 'Themselves' is the reflexive possessive pronoun that corresponds to 'they' or 'them', and is used when the subjects perform an action that affects themselves.
Step-by-step explanation:
The correct possessive pronoun to use in the sentence 'My grandparents have just bought ___________ a new electric car' is themselves. This pronoun is the reflexive form associated with 'they' or 'them', and it is used when the subjects of the sentence, in this case the grandparents, are also the objects, meaning they have performed an action that affects themselves. The word 'thereselves' is not a correct word in English grammar. So, the complete sentence should read, 'My grandparents have just bought themselves a new electric car.'