In the sentence "A mi hermanita le cayó un libro en el tobillo," the underlined word "tobillo" refers to "ankle" in English. Therefore, the correct answer is "ankle."
"A mi hermanita le cayó un libro en el tobillo."
"A mi hermanita" translates to "to my little sister."
"le cayó" translates to "fell on."
"un libro" means "a book."
"en el tobillo" means "on the ankle."
So, the entire sentence translates to "A book fell on my little sister's ankle." In this context, "tobillo" refers to the ankle, which is the part of the body where the book fell. Therefore, the correct answer is "ankle."