Your answer, "Jacob", is not correct.
The correct answer to
Who is Joseph's father (i.e. Jesus's grandfather)?
is
Heli
as shown by
Luke 3:23
which is
- KJV
- And Jesus himself began to be about thirty years of age, being (as was supposed) the son of Joseph, which was the son of Heli,
- NIV
- Now Jesus himself was about thirty years old when he began his ministry. He was the son, so it was thought, of Joseph, the son of Heli,