The pagan festival celebrating the winter solstice is called Yule.
Yule was a midwinter festival associated historically with Germanic-speaking peoples, including communities in Scandinavia and Anglo-Saxon England. Old Norse accounts describe feasting, ceremonial drinking, oath-taking, games, and gatherings during the Yule season. The festival’s exact date was not uniform, and scholars debate how closely every historical celebration was tied to the astronomical solstice.
As Christianity spread through northern Europe, Yule became closely blended with Christmas. “Yule” and related words remain names for Christmas or the Christmas season in several Scandinavian languages, and English still uses “Yuletide.” Modern Heathen and Wiccan groups may also celebrate Yule as a distinct religious observance at the winter solstice.
The other options refer to different seasonal festivals. Imbolc is associated with the beginning of spring, Beltane with the beginning of summer, and Samhain with the transition into winter in Celtic tradition. Yule is the conventional answer here, though pagan traditions are diverse and do not all use the same calendar or festival names.