Economics--------------my least favorite subject at school.

The verb "am" is the first-person singular form of "to be" and is used with the pronoun "I." In this case, the subject is "Economics," which requires a different verb form. Therefore, "am" does not agree with the subject and is incorrect.

The verb "are" is the plural form of "to be" and is used with plural subjects or with the pronouns "you," "we," and "they." Since "Economics" is treated as a singular noun, using "are" would lead to a grammatical mismatch, making it an unsuitable choice for this sentence.

The verb "been" is the past participle of "to be" and typically requires an auxiliary verb to form complete tenses, such as "has been" or "have been." In this context, "been" cannot stand alone as it does not convey the present state of the subject, thus rendering it incorrect for the sentence structure.