"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

1. The description provided – a trio of spacecraft flying in an equilateral triangle formation with sides one million kilometres long, using lasers to measure distances between them – precisely matches the mission concept for a space-based gravitational wave observatory. 2. Option (D), Evolved LISA (Laser Interferometer Space Antenna), now generally referred to as LISA, is a planned mission by the European Space Agency (ESA) designed exactly for this purpose: detecting gravitational waves using laser interferometry between three spacecraft separated by millions of kilometers. 3. Option (A), Voyager-2, is a single space probe for exploring the outer solar system and interstellar space. 4. Option (B), New Horizons, is a single space probe primarily for studying Pluto and the Kuiper Belt. 5. Option (C), LISA Pathfinder, was a technology demonstration mission for LISA, testing key components but involving only a single spacecraft and not the full triangular constellation. 6. Therefore, the experiment described refers to Evolved LISA.