It means that its always lined up
So,
The sentence is: "Once she left the store, my mother realizes she forgot her purse on the counter."
The phrase "One she left the store" is in the past tense.
In the next phrase, "my mother realizes", the tense shifts to present.
In the final phrase, "she forgot her purse on the counter", the verb tense shifts back to the past tense.
Therefore, option A is correct.
