The correct answer to the question "Which of these people would be David’s ancestor?" is:
his first relative to move to the U.S. in 1850.
An ancestor is defined as someone from whom a person is descended. This includes parents, grandparents, great-grandparents, and so on. David's grandmother is indeed a relative, but the question specifies an ancestor, which would only include people who directly contribute to his lineage, such as the first relative to move to the U.S. in 1850. The other options refer to relatives or friends that are not direct ancestors.