Harriet Tubman was a slave who got whipped as a child whenever she misbehaved, so she ran away and worked as a planter for a man named James Cook. Education was unheard of because of race issues.
Although "the girls" is in plural form, "each of the girls", the subject, is in singular form.
Therefore, the correct answer is c. her, for it corresponds with the subject.
Hope this helps~
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