Three parts of the business _ to be done.

Explanation

The correct answer is 'remain', as it correctly agrees with the plural subject 'Three parts of the business'. In English grammar, the subject and verb must agree in number, meaning a plural subject needs a plural verb. Here, 'remain' indicates that the action of being done applies to all three parts simultaneously.Option 'remains' is incorrect because 'remains' is singular and does not match the plural subject. Option 'is remain' incorrectly combines a singular verb with a plural action, leading to a grammatical error. Finally, option 'do remain' is not appropriate in this context as it introduces unnecessary auxiliary usage, making the sentence awkward and incorrect.