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

1. A

   Voyager-2

   Voyager-2. A single fly-by probe, not a million-km triangle.

2. B

   New Horizons

   New Horizons. Pluto fly-by.

3. C

   LISA Pathfinder

   LISA Pathfinder. A single technology demonstrator at L1, not the triangle.

4. D

   Evolved LISA

   Evolved LISA. Three spacecraft, equilateral triangle ~1 million km, laser interferometry for millihertz gravitational waves.

Summary. Official key is (d) eLISA / LISA. The million-kilometre laser triangle is the space-based gravitational-wave observatory. Pathfinder tested hardware; Voyager and New Horizons are planetary probes.