Introduction:
The author in the passage expresses his heavy-heartedness before leaving. There are several reasons that contribute to the author's heavy-heartedness. However, the correct answer is option D, which states that the author was heavy-hearted because there was still much to learn. Let's delve into the details to understand why this answer is correct.

Explanation:
Sympathy towards the disabled: Although the author shows empathy towards the disabled children in the passage, it is not the primary reason for his heavy-heartedness. The author does mention that he feels sorry for the disabled children and wishes he could do something to help them, but this is not the main cause of his heavy-heartedness.

Love for all children: While the author does express his love for children in general, this is not the primary reason for his heavy-heartedness. The author mentions that he loved all children, including those who were disabled, but this does not directly contribute to his heavy-heartedness.

Liking for Miss Beam: The author does mention that he liked Miss Beam, who was his teacher, but this is not the primary reason for his heavy-heartedness. The author expresses his gratitude towards Miss Beam for teaching him and showing kindness, but this does not directly cause his heavy-heartedness.

Still much to learn: The correct answer is option D because the author explicitly states that he was heavy-hearted because there was still much to learn. The passage mentions that the author had learned a great deal from Miss Beam, but he was sad to leave because there was still more knowledge to gain. The author states, "I was heavy-hearted at the thought of leaving Miss Beam and the others. There was so much more to learn."

Conclusion:
In conclusion, the author's heavy-heartedness before leaving is primarily due to the fact that there was still much to learn. Although the author expresses sympathy towards the disabled, love for all children, and liking for Miss Beam, these reasons are not directly responsible for his heavy-heartedness. The passage clearly states that the author was sad because he had not yet learned everything he wanted to, indicating that option D is the correct answer.