Detailed Explanation:

Answer: Option 4 — Evolved LISA

The description of three spacecraft flying in an equilateral triangle formation with sides one million kilometres long, using lasers for measurement, uniquely identifies the Evolved Laser Interferometer Space Antenna (eLISA) mission. This is a proposed space-based gravitational wave observatory designed to detect low-frequency gravitational waves by measuring tiny distance changes between the spacecraft using laser interferometry. Voyager and New Horizons are single-spacecraft exploration missions, while LISA Pathfinder was only a technology demonstrator for testing eLISA components, not the full mission itself.

📝 Short Notes: Gravitational Wave Detection Missions

• LISA (Laser Interferometer Space Antenna): Original concept for space-based gravitational wave detection proposed by ESA and NASA.
• Evolved LISA (eLISA): Redesigned version with three spacecraft in equilateral triangle formation, each separated by ~1 million km, using laser interferometry to detect gravitational waves.
• LISA Pathfinder (2015-2017): Technology demonstration mission that successfully tested drag-free flight and precision measurement technologies required for eLISA.
• Configuration: Three identical spacecraft maintain precise formation while measuring distance changes of picometers (10⁻¹² meters) caused by passing gravitational waves.
• Target: Detection of low-frequency gravitational waves (0.1 mHz to 1 Hz) from sources like merging supermassive black holes, binary star systems, and early universe events.
• Ground-based counterpart: LIGO (Laser Interferometer Gravitational-Wave Observatory) detects high-frequency gravitational waves on Earth.