The event that marks the end of stage 1 of a normal delivery is the dilation of the cervix to 10 cm, which is needed for the baby to move into the birth canal.
We call normal delivery the action of giving birth naturally to a child, without medical intervention. If Sue told Teresa she has reached the end of stage 1 of normal delivery, she means that her cervix has dilated up to 10 cm and has become thinner. Such process is what allows the baby to move and get in position for birth.
Therefore, we can conclude that the best answer for this question is "dilation of the cervix to 10 cm." The answer provided above is correct.
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