How did gentiles become part of Abraham's seed?

Abraham had a son with Hagar, his wife’s handmaiden, named Ishmael. He is at the start of the Muslim faith.

Then Abraham had his son Isaac with his wife Sarai turned Sarah. He is at the start of the Judeo-Christian faith.

There are other ancient books that say after Sarah died, Abraham had sons with Keturah named Zimran, Jokshan, Medan, Midian, Ishbak, and Shuah.

The correct answer is b