The purpose for which Yung Wing would include this explanation in his text is to D. To illustrate how common it was to have a Chinese child educated in English.
What is the purpose?
Option D best aligns with the provided excerpt by showing how Chinese children receiving English education, exemplified by Mrs. Gutzlaff's school, was already happening at a young age. This sets the stage for Yung Wing's own experience and the growing trend of Western education in China.
For option A, while highlighting the involvement of missionaries is true, the specific mention of Mrs. Gutzlaff's early arrival and the opening of both girls' and boys' schools suggests a broader context than just attributing his education to them.