The familiar ache refers to a feeling of pain or discomfort that is commonly experienced and recognized by an individual. In the given options, the correct answer is "her childhood fear of losing her mother", which suggests that the protagonist of the story is familiar with the ache of anxiety and worry related to her mother's health.

Explanation:

- Childhood fear: The protagonist's fear of losing her mother is rooted in her childhood experiences, which have shaped her emotional responses and perceptions of the world around her.
- Losing her mother: The fear of losing someone close is a common source of emotional pain and distress, and the protagonist's anxiety about her mother's health reflects this familiar ache.
- Recognition: The use of the phrase "familiar ache" suggests that the protagonist is not alone in experiencing this feeling, and that it is something that is recognized and understood by others who have gone through similar experiences.
- Emotional impact: The familiar ache is not just a physical sensation, but also an emotional one, as it reflects the protagonist's deep love and concern for her mother.

In summary, the familiar ache in this context refers to the protagonist's childhood fear of losing her mother, which is a common source of emotional pain and distress that is recognized and understood by others who have gone through similar experiences.