Final answer:
The correct answer is option d. Mrs. Alida Slade is described as a mysterious and reserved lady, who is difficult to know deeply and has an enigmatic presence.
Step-by-step explanation:
Based on the description provided, Mrs. Alida Slade is best described as (d) a mysterious and reserved lady. The passage suggests she often wore a mask by instinct and was hard to know on a deeper level, indicating a reserved nature. She is also likened to a shadow or a phantom, which gives a sense of mystery.
Moreover, her character is remembered by a 'lurid flash,' further enhancing the enigmatic quality attached to her persona.