Which technological achievement was demonstrated in ISRO’s reusable launch vehicle landing experiment?

The correct option is Autonomous precision landing.

Explanation:

In ISRO's Reusable Launch Vehicle Autonomous Landing Mission (RLV LEX), the primary technological achievement demonstrated was the autonomous precision landing of a winged space vehicle. During this mission, the vehicle was carried to an altitude by a helicopter and released, after which it autonomously navigated, performed approach and landing maneuvers, and landed precisely on a runway under real-world conditions without human intervention.

Let us analyze the given options:

1. Deep-space docking: Refers to connecting two spacecraft in deep space, which was not the focus of the RLV LEX experiment.
2. Autonomous precision landing: Correct. This experiment demonstrated high-speed, unmanned autonomous precision landing capability on a runway.
3. Nuclear propulsion: Refers to using nuclear reactions for thrust, which is an unrelated concept.
4. Underwater recovery: Involves retrieving spacecraft modules from water bodies, whereas RLV LEX demonstrated a precise horizontal runway landing.