A.I. Explanation:

Correct answer: at the same time

Explanation: The phrase 'at the same time' serves as an idiomatic expression that indicates simultaneous occurrences. In this context, it effectively conveys the paradox of teens being clever and reckless simultaneously, maintaining grammatical coherence and aligning with the overall sentiment of the sentence.

Meaning: 'At the same time' means occurring simultaneously or concurrently.

Wrong options:

• buffoons: 'Buffoons' refers to foolish or silly people, which does not fit the context of describing teens' behavior.
• inventing: 'Inventing' is a gerund that implies creating something new, which does not relate to the simultaneous nature of being clever and reckless.
• exploring: 'Exploring' is a gerund that suggests investigation or examination, which does not fit the context of simultaneous traits.
• development: 'Development' refers to the process of growth or progress, which does not convey the idea of simultaneous traits in this context.
• school: 'School' is a noun that refers to an educational institution, which does not fit the context of describing the simultaneous nature of behaviors.

Correct answer: exploring

Explanation: The word 'exploring' is a gerund that indicates the action of investigating or examining. It fits grammatically as it follows the verb 'have been,' maintaining the present perfect continuous tense, which describes an ongoing action.

Meaning: 'Exploring' means to investigate or examine something in detail.

Wrong options:

• buffoons: 'Buffoons' does not fit as it is a noun referring to foolish people, which does not relate to the action of studying the brain.
• inventing: 'Inventing' implies creating something new, which does not align with the context of studying or examining the brain's structure.
• at the same time: This phrase does not fit as it does not represent an action but rather a temporal relationship, which is not appropriate here.
• development: 'Development' is a noun that refers to growth or progress, which does not fit the context of an ongoing action of investigation.
• school: 'School' is a noun that refers to an educational institution and does not fit the context of the action being described.

Correct answer: development

Explanation: The term 'development' is a noun that refers to the process of growth or advancement. It fits contextually as it describes the ongoing changes and maturation occurring during adolescence, aligning with the discussion of the teenage brain.

Meaning: 'Development' refers to the process of growth or progress, particularly in a biological or psychological context.

Wrong options:

• buffoons: 'Buffoons' does not fit as it refers to foolish people, which is unrelated to the concept of growth or maturation.
• inventing: 'Inventing' implies the creation of something new, which does not relate to the process of maturation or growth in the context of the brain.
• exploring: 'Exploring' is a gerund that refers to the action of investigation, which does not fit as a noun describing a state of being or process.
• at the same time: This phrase does not fit as it does not represent a state of growth or development, but rather a temporal relationship.
• school: 'School' is a noun that refers to an educational institution and does not relate to the concept of growth or development.