"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

Answer: D. The description matches EVOLVED LISA (eLISA), the space-based gravitational-wave observatory. LISA (Laser Interferometer Space Antenna) and its successor concept EVOLVED LISA (eLISA, also called LISA) are a planned SPACE-BASED GRAVITATIONAL-WAVE OBSERVATORY, a joint mission of the EUROPEAN SPACE AGENCY (ESA) and NASA. eLISA is now part of ESA's Cosmic Vision programme as the L3 mission, expected to launch in the mid-2030s. KEY FEATURES described in the quotation: 1. THREE SPACECRAFT IN FORMATION: eLISA consists of three identical spacecraft arranged in an equilateral triangle formation, orbiting the Sun in a heliocentric orbit, trailing Earth at roughly 60 degrees behind. 2. ONE MILLION KILOMETRES BASELINE: each side of the triangle is around 2.5 million kilometres in the latest plan (earlier eLISA proposals had 1 million km arms; LISA Pathfinder demonstrated the precision technology). The PYQ uses the figure 1 million kilometres, consistent with the earlier eLISA configuration. 3. LASERS BETWEEN CRAFT: free-flying TEST MASSES inside each spacecraft are tracked by laser interferometry from one craft to another. Tiny changes in arm length (of the order of picometres) caused by passing gravitational waves are measured. WHAT eLISA DETECTS: low-frequency GRAVITATIONAL WAVES in the milliHertz band (around 0.1 milliHz to 1 Hz), which is below the band of ground-based LIGO/Virgo (around 10 Hz to 10 kHz). Sources detectable by eLISA include: 1. MERGERS OF SUPERMASSIVE BLACK HOLES at galactic centres. 2. EXTREME MASS RATIO INSPIRALS (a stellar-mass black hole orbiting a supermassive one). 3. COMPACT GALACTIC BINARIES (close white dwarf and neutron star pairs in our galaxy). 4. POSSIBLE STOCHASTIC GRAVITATIONAL WAVES from the early universe. Why other options are WRONG: (A) Voyager 2 is a NASA probe launched 1977, currently in interstellar space. Not a gravitational-wave observatory. (B) New Horizons (NASA, 2006) flew past Pluto in 2015 and explored Kuiper Belt object Arrokoth in 2019. Planetary science mission, not gravitational waves. (C) LISA PATHFINDER (ESA, 2015-17) was a SINGLE-SPACECRAFT TECHNOLOGY DEMONSTRATOR for the eLISA mission, validating drag-free flight and laser interferometry of test masses. Not the three-spacecraft observatory described. Source: European Space Agency LISA / eLISA mission documentation; NASA LISA project page; Nature reviews on gravitational-wave astronomy.