Which one of these is an example of an open-ended question

Open-ended questions are questions which need to be answered with more than a "yes" or a "no".

"Did you grandparents live with you while you were growing up?"
"Yes/No."

"Have any of your relatives traveled out of the country?"
"Yes/No."

"Did your father run for mayor in your home town?"
"Yes/No."

"How did your ancestors get their covered wagons across the rivers?"
[Here you would need to elaborate and answer with more than a "yes" or a "no".]

D. How did your ancestors get their covered wagons across the rivers?