Concept: Section 303 IPC provided mandatory death sentence for a person who committed murder while undergoing sentence of imprisonment for life.
Step 1: Understand the constitutional issue.
Mandatory death penalty removed judicial discretion and treated all cases alike without considering individual circumstances.
\[
\text{Mandatory death penalty} \Rightarrow \text{Problem under Articles 14 and 21}
\]
Step 2: Identify the case.
In Mithu v. State of Punjab, the Supreme Court held Section 303 IPC unconstitutional.
\[
\text{Section 303 IPC ultra vires} \Rightarrow \text{Mithu v. State of Punjab}
\]
Step 3: Select the correct option.
Therefore, the correct answer is Mithu v. State of Punjab.