Answer:
A. Exposition.
Step-by-step explanation:
In any narrative text, the exposition is the descriptive details about what the story is, the background details of what the story will be or it may even be about the setting or details about the past. It provides the background details which may serve an important aspect in the whole plot or narrative.
Helen Keller's The Story of My Life is a narrative about how she overcame her disability of being blind and deaf. In the given excerpt, she recounts the day her teacher Anne Sullivan came to her. Keller mentioned that that day was "the most important day I remember in all my life", asserting how important it was for her to get a teacher like Miss Sullivan. This part of the plot is the exposition or the background information of what the whole plot will be about.