Step 1: Understanding jus cogens.
The term jus cogens refers to fundamental principles of international law that are binding on all states, without exception.
Step 2: Nature of jus cogens norms.
They are peremptory norms recognized by the international community as norms from which no derogation is permitted, even by mutual agreement between states. Examples: prohibition of genocide, slavery, torture, and crimes against humanity.
Step 3: Elimination of incorrect options.
(a) Incorrect — a UNGA resolution is not binding like a jus cogens norm.
(b) Incorrect — judicial decisions are not per se jus cogens norms.
(d) Incorrect — treaty obligations can be altered by consent, unlike jus cogens norms.
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\boxed{(c)}
\]