Concept: Austin and Kelsen both explained law from different positivist approaches. Austin used the concept of sovereign, while Kelsen used the concept of Grundnorm.
Step 1: Understand Austin's view.
Austin defined law as a command of the sovereign. For Austin, the sovereign is a determinate human superior who is habitually obeyed by the people.
\[
\text{Austin} \Rightarrow \text{Determinate human superior}
\]
Step 2: Understand Kelsen's view.
Kelsen gave the Pure Theory of Law. According to him, the validity of all legal norms ultimately depends on a basic norm called the Grundnorm.
\[
\text{Kelsen} \Rightarrow \text{Grundnorm}
\]
Step 3: Match the concept with the jurist.
The term Grundnorm is directly associated with Hans Kelsen.
Therefore, the correct answer is Kelsen.