Which question led Jesus to tell the parable of the Good Samaritan?

Answer: Who is my neighbor?

An expert in the law, wanting to justify himself, asked, "And who is my neighbor?" Jesus answered with the parable.

“But wanting to justify himself, he asked Jesus, “And who is my neighbor?” Jesus took up this question and said, “A man was going down from Jerusalem to Jericho when he fell into the hands of robbers. They stripped him, beat him, and went away, leaving him half dead.”— Luke 10:29-30 (BSB)