Question: which  North Germanic tribe did Horik I reigned as sole King of

Context paragraphs:
Horik I: Horik I (died 854) reigned as sole King of the Danes from 827 to his violent death in 854.  His reign was marked by Danish raids on the Franco-German empire of Louis the Pious, son and successor of Charlemagne.

Quadi: The Quadi were a Suebian Germanic tribe who lived approximately in the area of modern Moravia in the time of the Roman empire.  The only known information about the Germanic tribe the Romans called the 'Quadi' comes through reports of the Romans themselves, whose empire had its border on the River Danube just to the south of the Quadi.  They associated the Quadi with their neighbours the Marcomanni, and described both groups as having entered the region after the Celtic Boii had left it deserted.  The Quadi are thought to have been an important part of the Suebian group who crossed the Rhine with the Vandals and Alans in the 406 Crossing of the Rhine, and later founded a kingdom in northwestern Iberia.

Northwest Germanic: Northwest Germanic is a proposed grouping of the Germanic languages, representing the current consensus among Germanic historical linguists.  It does not challenge the late 19th-century tri-partite division of the Germanic dialects into North Germanic, West Germanic and East Germanic, but proposes additionally that North and West Germanic (i.e. all surviving Germanic languages today) remained as a subgroup after the southward migration of the East Germanic tribes, only splitting into North and West Germanic later.  Whether this subgroup constituted a unified proto-language, or simply represents a group of dialects that remained in contact and close geographical proximity, is a matter of debate.  The date by which such a grouping must have dissolved—in that innovations ceased to be shared—is also contentious, though it seems unlikely to have persisted after 500 AD, by which time the Anglo-Saxons had migrated to England and the Elbe Germanic tribes had settled in Southern Germany.

Chamavi: The Chamavi were a Germanic tribe of Roman imperial times whose name survived into the Early Middle Ages.  They first appear under that name in the 1st century AD "Germania" of Tacitus as a Germanic tribe that lived to the north of the Lower Rhine.  Their name probably survives in the region today called Hamaland, which is in the Gelderland province of the Netherlands, between the IJssel and Ems rivers.

Marsi (Germanic): The Marsi (German: "Marser" ) were a small Germanic tribe settled between the Rhine, Ruhr and Lippe rivers in northwest Germany.  It has been suggested that they were a part of the Sugambri who managed to stay east of the Rhine after most Sugambri had been moved from this area.  Strabo describes the Marsi as an example of a Germanic tribe who were originally from the Rhine area, now the war-torn Roman frontier, but had migrated deep into Germania.

Old Norse religion: Old Norse religion developed from early Germanic religion during the Proto-Norse period, when the North Germanic peoples separated into a distinct branch of the Germanic peoples.  It was displaced by Christianization of the North Germanic peoples.  Scholars reconstruct aspects of North Germanic religion by historical linguistics, archaeology, toponymy, and records left by North Germanic peoples, such as runic inscriptions in the Younger Futhark, a distinctly North Germanic extension of the runic alphabet.  Numerous Old Norse works dated to the 13th century record Norse mythology, a component of North Germanic religion.

Swedes (Germanic tribe): The Swedes (Swedish: "svear" ; Old Norse: "svíar / suar" (probably from the PIE reflexive pronominal root *s(w)e, "one's own [tribesmen/kinsmen]"; Old English: "Sweonas") were a North Germanic tribe.

Danes (Germanic tribe): The Danes were a North Germanic tribe inhabiting southern Scandinavia, including the area now comprising Denmark proper, during the Nordic Iron Age and the Viking Age.  They founded what became the Kingdom of Denmark.  The name of their realm is believed to mean "Danish March", viz.  "the march of the Danes" in Old Low German, referring to their southern border zone between the Eider and Schlei rivers.

Swedish language: Swedish (   ] ) is a North Germanic language, spoken natively by more than 9 million people predominantly in Sweden (as sole official language) and parts of Finland, where it has equal legal standing with Finnish.  It is largely mutually intelligible with Norwegian and Danish.  Swedish is a descendant of Old Norse, the common language of the Germanic peoples living in Scandinavia during the Viking Era.  It is currently the largest of the North Germanic languages by number of speakers.

Geats: The Geats ( , , or ) (Old English: "gēatas" ] ; Old Norse: "gautar" ] ; Swedish: "götar" ] ), and sometimes "Goths") were a North Germanic tribe and are a current culture group inhabiting what is now Götaland ("land of the Geats") in southern Sweden.  The name of the Geats also lives on in the Swedish provinces of Västergötland and Östergötland, the Western and Eastern lands of the Geats, also exonymed (however not at "Geats") and endonymed in many other toponyms.

Supporting paragraphs: Horik I Danes (Germanic tribe)
Answer: The Danes