1) Explanation

Let's break down the sentence to understand the context and the meaning required for the blank:
- "The girl's composure was astounding given her youthfulness; $\qquad$ and selfpossessed,..."
- The sentence begins by highlighting the girl's "composure" despite her youth. "Composure" means a state of being calm and in control of oneself.
- The blank, followed by "and self-possessed," needs a word that reinforces this idea of being calm, controlled, dignified, or proper. "Self-possessed" means calm, confident, and in control of one's feelings.
- "...she clearly had no predilection for confusing exhilaration with contentment."
- This further emphasizes her maturity and clear-headedness. She doesn't mistake fleeting excitement ("exhilaration") for deeper, lasting satisfaction ("contentment"). This shows wisdom and an understanding of her own emotions.

Therefore, the blank needs a word that describes a calm, dignified, or properly behaved demeanor, fitting with "composure" and "self-possessed" and indicative of maturity.

Let's evaluate the given options:
- A. nonplussed: Surprised and confused so much that one is unsure how to react. This is the opposite of composure.
- B. demure: (Of a woman or her behavior) reserved, modest, and shy. A demure person is often composed in their quietness and modesty. This fits the idea of a calm and controlled demeanor.
- C. contrite: Feeling or expressing remorse or penitence; affected by guilt. This is unrelated to composure or maturity of understanding.
- D. decorous: In keeping with good taste and propriety; polite and restrained. A decorous person exhibits dignity, proper behavior, and self-control, all of which contribute to composure and self-possession.
- E. lethargic: Sluggish and apathetic. This is the opposite of being composed and clear-headed.
- F. apathetic: Showing or feeling no interest, enthusiasm, or concern. This describes a lack of emotion, but not necessarily composure or self-possession in the positive sense implied.

Conclusion:

Both "demure" and "decorous" describe a composed, restrained, and proper demeanor.
- "Demure" emphasizes modesty and quietness, which aligns with composure.
- "Decorous" emphasizes propriety and dignity, which also aligns strongly with composure and self-possession.

Given that two answers are required and they describe similar aspects of a composed and mature individual, they form the correct pair.

The final answer is $\(\mathrm{B}, \mathrm{D}\)$.