Explanation:
The correct answer to the given question is "must be written". This is because the sentence suggests a requirement or obligation that all compositions should be written in ink. Let's break down the explanation into headings and key points for better understanding.

Understanding the Requirement
- The question states that "all of our compositions" should be written in ink.
- The use of the word "should" indicates that it is necessary or mandatory to write compositions in ink.
- The teacher has set this requirement and will not accept papers written in pencil.

The Meaning of "Must"
- "Must" is a modal verb that expresses obligation or necessity.
- It is used to indicate that something is required or mandatory.
- In this context, "must" emphasizes the teacher's requirement for compositions to be written in ink.
- It implies that there is no alternative or flexibility in using pencil; ink is the only acceptable medium.

Forming the Correct Answer
- The correct answer is "must be written" because it captures the obligation and requirement expressed in the sentence.
- "Must" emphasizes the necessity, and "be written" indicates the action of writing compositions.
- The phrase "must be written" aligns with the meaning conveyed by the sentence.
- It correctly reflects the teacher's requirement for all compositions to be written in ink.

Using "Must Be" as a Construction
- "Must be" is a common construction used to express obligation or necessity.
- It is often followed by a verb in its past participle form.
- In this case, the verb "written" is the past participle form of the verb "write".
- Using "must be" followed by the past participle form of the verb correctly conveys the requirement that compositions should be written in ink.

Conclusion
- The requirement stated in the sentence is that all compositions must be written in ink.
- The correct answer is "must be written" because it accurately captures the obligation and necessity indicated in the sentence.
- By using "must be" followed by the past participle form of the verb "write", the correct answer reflects the teacher's requirement for using ink instead of pencil.