Answer:
"The mildness of her temper had hardly allowed her to impose any restraint" means the following about Miss Taylor:
D. Miss Taylor's gentle nature prevented her from making a lot of rules for Emma.
Step-by-step explanation:
Miss Taylor's temper was mild, which means her usual state of mind was serene, tranquil, light and good-natured.
Miss Taylor's temper had hardly allowed her to impose any restraint, which means her usual mindset did not match the idea of keeping Emma under absolute control.
All of the above can be said with the words we see in letter D: Miss Taylor's gentle nature prevented her from making a lot of rules for Emma.