Which legendary Norse king was the grandfather of Sigurd?

Völsung was the legendary Norse king and grandfather of Sigurd.

The family line runs from Völsung to his son Sigmund, and then to Sigurd, the dragon-slayer. Sigurd’s mother was Hjördís, but his paternal descent from Völsung gives the heroic clan its name. The Völsungs are the central family of the Völsunga saga, a late-13th-century Icelandic prose retelling of older Germanic heroic traditions.

Völsung’s own ancestry is extraordinary. In the saga, he is the son of Rerir and a descendant of Odin through Sigi. Völsung fathers the twins Sigmund and Signy, whose family tragedies shape the saga before Sigurd’s birth. After Sigmund is mortally wounded, Hjördís preserves fragments of his broken sword, which are later reforged for Sigurd as Gram.

The distractors represent different relationships or figures: Odin is an ancestor, Hjördís is Sigurd’s mother, and Sigmund is his father. Thus Völsung is the single canonical answer.