Which type of opinion becomes law in Supreme Court decisions?

Regardless of the US Supreme Court's decision, there is no guarantee any type of opinion will be written. Per curiam (unsigned) decisions -- which are reasonably common -- seldom include full written opinions. About 20% of the US Supreme Court's decisions lack any written opinion; many simply affirm a lower court's decision without comment, or remand a case to a lower court for further action in light of a particular cited decision.

Other cases may include only a majority opinion (the "opinion of the Court"), because there is no requirement for the minority to write a dissenting opinion. There are many examples of split-votes that lack dissenting opinions, and some even include a concurring opinion from a member of the majority.

If this is a homework question, the expected (but incorrect) answer is majority and dissenting.