Final answer:
The Kellers are considered lucky parents because their daughter overcame a disability.
Step-by-step explanation:
The Kellers are considered lucky parents because their daughter overcame a disability (Option D). This is evident from the statement, 'She is a young woman of good ability, studious habits, and a very fine character.' Despite her disability, she was one of the best students and had a broad and thorough training. The fact that the Kellers were fortunate enough to have a daughter who achieved such success despite her disability can be seen as lucky.