The answer is (c), Mrs. Harling. Antonia looks for a model in how to run a household and bring up her children in her memories of Mrs. Harling.
When Jim Burden visits Antonia on the farm she and her husband own, he notes her happiness, and mentions that she "ought never to have gone to town." Antonia replies,
"Oh, I'm glad I went! I'd never have known anything about cooking or housekeeping if I hadn't. I learned nice ways at the Harlings', and I've been able to bring my children up so much better. Don't you think they are pretty well-behaved for country children? If it hadn't been for what Mrs. Harling taught me, I expect I'd have brought them up like wild rabbits" (Book V, Chapter 1).
The Harlings were neighbors of the Burdens when Jim moved to town with his grandparents. Mrs. Harling, a hearty, down-to-earth woman, hired Antonia to help with the house and the children, and the two got on very well. Although conflict between Antonia and Mr. Harling eventually resulted in the young girl leaving the household, during her time there, Antonia worked hard and learned a lot. The arrangement was mutually beneficial while it lasted, and Antonia remembers and appreciates the skills she acquired from Mrs. Harling during those days for the rest of her life.