"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

"The experiment will employ a trio of spacecraft flying in formation…

Answer

(a) Voyager-2: It is a deep-space planetary probe launched in 1977 for outer planet exploration. It has nothing to do with a triangular laser interferometer formation. Verdict: Incorrect.
(b) New Horizons: This mission was sent to Pluto and the Kuiper Belt. It is not a three-spacecraft laser interferometry experiment. Verdict: Incorrect.
(c) LISA Pathfinder: It was a technology demonstrator for space-based gravitational-wave detection, but it was a single spacecraft, not the full million-km triangular constellation. Verdict: Incorrect.
(d) Evolved LISA: eLISA/LISA refers to the proposed space-based gravitational-wave observatory using three spacecraft in triangular formation with laser links over about one million km arms. Verdict: Correct.