The sentence is imperative meaning the subject is (you)
“ teens need adequate sleep and REM sleep to rebuild long term memory for learning and , focus attention for driving and maintaining health and fighting obesity.”
I think it would be a. formal
Explanation:
Infinitive phrases include infinitives. ... Examples include, “to walk,” “to read,” or “to eat.” Infinitives can act as nouns, adjectives, or adverbs. As a noun, they might act as the subject of the sentence. For example, “To travel is the only thing on her mind.” As an adjective, they'll modify a noun.
All, namely because it depends on how it’s written or produced.