The correct option to complete the sentence "He sent the children out to play .... he might be alone" is B. so that. Step-by-Step Explanation:

1. Understanding the Sentence Structure:
2. The sentence is structured to show a cause-and-effect relationship. The first part of the sentence ("He sent the children out to play") indicates an action taken by the subject (he), and the second part ("he might be alone") suggests a purpose or reason for that action.
3. Analyzing the Options:
4. A. because: This word is used to indicate a reason. If we used "because," the sentence would read: "He sent the children out to play because he might be alone." This implies that the reason for sending the children out is that he might be alone, which changes the intended meaning. The focus shifts from wanting to be alone to explaining why he sent them out.
5. B. so that: This phrase is used to express purpose. The sentence would read: "He sent the children out to play so that he might be alone." This indicates that the purpose of sending the children out is to achieve the state of being alone. This aligns perfectly with the intended meaning of the sentence.
6. C. yet: This word is a conjunction that indicates contrast. If we used "yet," the sentence would read: "He sent the children out to play yet he might be alone." This construction is awkward and does not logically connect the two clauses. It suggests a contradiction that does not fit the context.
7. D. since: This word can indicate time or reason. If we used "since," the sentence would read: "He sent the children out to play since he might be alone." This implies that he sent them out because he might be alone, which again shifts the focus away from the purpose of wanting to be alone.
8. Why Option B is Correct:
9. "So that" clearly indicates that the action of sending the children out is intended to achieve the result of being alone. It maintains the original intent of the sentence, which is to express a purpose.
10. Common Pitfalls:
11. Students often confuse "because" and "so that." Remember that "because" explains a reason for an action, while "so that" indicates the purpose of an action.
12. Misunderstanding the use of "yet" can lead to awkward sentence constructions. It’s important to recognize that "yet" introduces contrast, which is not suitable in this context.
13. "Since" can be tricky because it can mean both "because" and "from a certain time." In this case, it does not convey the intended purpose.

Revision Summary:

• The correct answer is B. so that because it indicates purpose.
• "Because" explains a reason, which changes the meaning of the sentence.
• "Yet" introduces contrast and does not fit the context.
• "Since" can imply reason or time, but does not convey the intended purpose of the action.

By understanding the nuances of these conjunctions, you can better determine the correct word to complete sentences in English.