“The experiment will employ a trio of spacecraft flying in formation in the shape of an equilateral triangle that has sides one million kilometres long, with lasers shining between the craft,” The experiment in question refers to

The correct answer is D, Evolved LISA (eLISA). It is the ESA concept for a space-based gravitational wave observatory built from three spacecraft flying in a near-equilateral triangle, with laser beams exchanged between them to measure tiny changes in distance caused by passing gravitational waves. The arm length of one million km was the figure in the original eLISA proposal; the adopted successor mission, LISA, now plans arms of about 2.5 million km, so the exact dimension has changed.

• A) Voyager-2: A single NASA probe launched in 1977 to study the outer planets and the heliosphere. It does not fly in formation with other craft.
• B) New Horizons: A single spacecraft that flew past Pluto in 2015 and later visited a Kuiper Belt object. No formation flying or inter-craft lasers are involved.
• C) LISA Pathfinder: An ESA-NASA technology demonstrator launched in 2015. It carried two test masses inside one spacecraft to prove the laser and drag-free techniques, and it was not a three-craft observatory. It was the precursor to the full mission.

The core concept is space-based gravitational wave astronomy, which can detect low-frequency waves that ground detectors like LIGO cannot. For Mains GS-III (science and technology), it links to how India and other nations are investing in frontier physics and precision space technology.

Trap: Aspirants pick LISA Pathfinder just because the name matches. The question describes a three-craft observatory, not a single-craft test mission.