In Japan, spring rolls are often served with soy sauce or karashi mustard.
Japanese spring rolls are known as harumaki, written 春巻き and literally associated with a “spring roll.” They are typically cylindrical fried pastries filled with ingredients such as vegetables, pork, shiitake mushrooms, or glass noodles. Soy sauce provides a salty, savory dip, while karashi adds a sharp heat that complements the crisp wrapper and mild filling.
Karashi is a Japanese mustard condiment made from powdered mustard seeds mixed with water. It is hotter and more pungent than the yellow mustard commonly served with Western hot dogs, and it is used in foods including oden, tonkatsu, natto, and certain dumplings. It may be served as a paste or incorporated into sauces.
Wasabi is the tempting wrong answer because it is also a famously pungent Japanese condiment, but it is a different plant-based seasoning and is most strongly associated with sushi and sashimi. Ponzu is a citrus-seasoned sauce, while miso is a fermented soybean paste. None is the mustard named in this spring-roll pairing.