"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 experiment described refers to Evolved Laser Interferometer Space Antenna (eLISA), a project designed to detect gravitational waves.

eLISA involves three spacecraft flying in an equilateral triangle formation, with sides of one million kilometers.

Lasers are used to measure the minute changes in distance between freely falling test masses within the spacecraft, which would indicate the presence of a gravitational wave.

This concept builds upon the success of the LISA Pathfinder experiment.

Therefore, option (d) is the correct answer.

Sources:

• ESA - LISA