Which answer best describes the verb tenses in the sentence?
once she left the store, my mother realizes she forgot her purse on the counter.
a. the verb tenses are consistent.
b. the verb tenses shift.

Once she left the store, my mother realizes she forgot her purse on the counter.

Left is past tense, but realizes is present tense.

The way to correct the sentence would be:

Once she left the store, my mother realized she forgot her purse on the counter.

Your answer is 
B.The verb tenses shift.