Step 1: Spot the core problem.
A pension scheme treats employees who retired before a cut-off date differently from those who retired after it. The excluded ones say this is arbitrary. We must find which constitutional rule applies.
Step 2: What does arbitrary classification point to?
Whenever a rule divides people into groups and someone says the division is unfair, the question of equality comes up. Equality is protected by Article 14 of the Constitution.
Step 3: Understand Article 14.
Article 14 guarantees equality before the law. It allows the State to make classifications, but only reasonable ones. A valid classification needs an intelligible differentia (a clear basis) and a rational nexus (a sensible link) to the goal it wants to achieve.
Step 4: Apply it here.
A cut-off date creates two classes of retirees. The court will test whether this date is reasonable or arbitrary. If it is arbitrary, it breaks Article 14. So the heart of the issue is classification under Article 14.
Step 5: Rule out the other options.
The doctrine of eclipse and doctrine of severability deal with what happens to a law that clashes with fundamental rights, not with whether a classification is fair. Legislative competence is about which legislature can make the law, which is not the issue here.
Step 6: Confirm the right choice.
The challenge is purely about unequal treatment, so it directly attracts Article 14 and the test of reasonable classification.
Step 7: Final answer.
\[ \boxed{\text{Article 14 and the principle of classification}} \]