"The experiment will employ a trio of spacecraft flying in formation in the shape of an equilateral triangle…
"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
Voyager-2
New Horizons
LISA Pathfinder
Evolved LISA
Solution
Evolved Laser Interferometer Space Antenna (eLISA) is a planned ESA/NASA mission using three spacecraft in an equilateral-triangle formation (millions of km on a side) with laser interferometry to detect gravitational waves. LISA Pathfinder (2015) was the technology demonstrator. Answer: (d).