The ball-shaped decorations that go on a Christmas tree are called ornaments. More specifically, they are often called Christmas ornaments, baubles, or Christmas balls.
The tradition of decorating evergreen trees became widely popular in German-speaking Europe. Early decorations included apples, nuts, sweets, and candles. By the 19th century, German glassmakers—especially in Lauscha, Germany—were producing delicate glass balls that helped inspire the modern tree bauble.
“Bauble” is especially common in British English, while “ornament” is the broader North American term. Not every tree ornament is ball-shaped: stars, figurines, garlands, lights, ribbons, and handmade decorations also count as ornaments. A fair quizmaster would therefore accept “ornaments” as the standard answer.
The familiar shiny glass balls are now commonly made from plastic for safety and durability. They may be plain, painted, glittered, or personalized with names and dates.