Economics ____________ my least favorite subject at school.

The correct choice is "is" because the subject "Economics" is a singular noun, and it refers to a specific field of study. In English, singular subjects require singular verbs. The verb "is" appropriately agrees with the singular noun "Economics," indicating that it is continuously, or in a general context, the least favorite subject for the speaker.

Using "are," "am," or "been" would disrupt the subject-verb agreement and create a grammatically incorrect sentence. "Am" is used with the pronoun "I," while "are" is used with plural subjects or with "you." "Been" is the past participle of the verb "be" and is not suitable in this context, as it does not convey the current state of being least favorite. Hence, "is" is the only option that fits correctly in this sentence structure.