In the early Middle Ages, the term "mercenaries" often referred to soldiers for hire. However, none of the options provided—Slavs, Magyars, Muslim Arabs, or Normans—are direct alternate names for "mercenaries."
If you are looking for which group historically engaged in mercenary activities or could fit the definition loosely, it would be more context-dependent. For instance, the Normans are often noted for their mercenary practices as they became involved in various conflicts across Europe.
If I had to choose one from your list that could be associated with the concept of mercenaries, it would be Normans.