If two actions take place in the past, one after the other, the 1st action will be in the Past Perfect Tense and the 2nd action will be in the Simple Past Tense.
The formula for using the Past Perfect Tense is: Subject + had + V3 + object.
Examples,
I had written notes on various topics before she came.
He had studied well before he came to the class.
According to the explanation and examples, 'had seen' will be used in the blank.
Complete Sentence: I had seen her before she stopped her car.