Final answer:
Based on the Pew Research Center, about one-third of women who had a child with an unmarried partner formed a new married or cohabiting union within five years.
Step-by-step explanation:
The Fragile Families and Child Wellbeing Study provides insights into family structures and dynamics in the U.S. after the birth of a child to unmarried couples. The data indicates significant trends in relationship status changes following the birth of a child. Upon reviewing the various sources and considering the context provided, the answer to the student's question is c) one-third. It is reported by the Pew Research Center that about one-third of U.S. children are living with an unmarried parent. This strongly suggests that a corresponding proportion of women who had a child with an unmarried partner formed a new married or cohabiting union within five years.