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 option is Evolved LISA.

The description refers to a space-based gravitational wave observatory. The experiment designed to employ a trio of spacecraft flying in formation in the shape of an equilateral triangle with sides of one million kilometres long, using lasers shining between them, is the Laser Interferometer Space Antenna (LISA), which is often referred to in its current design phase as Evolved LISA (eLISA) or simply LISA.
Let us look at why the other options are incorrect:
- Voyager refers to Voyager 1 and 2, which are single space probes sent to study the outer solar system and interstellar space.
- New Horizons is a single spacecraft mission that performed a flyby of Pluto and Kuiper belt objects.
- LISA Pathfinder was a single spacecraft mission launched to test the critical technologies required for the full LISA mission, but it did not consist of three spacecraft flying in a million-kilometer triangle formation.
Therefore, the full-scale mission described is Evolved LISA.