• The experimental research on black holes using gravitational waves expanded following the second detection of gravitational waves by the LIGO detector.
• After the success of the LISA Pathfinder experiment, the evolved Laser Interferometer Space Antenna (eLISA) project is a plan of setting into space three spacecraft, a mother and two daughter spacecraft, which will fly in a triangular formation, trailing the earth in its orbit around the sun at a distance of over 50 million km.
• Each arm of the imaginary triangle, from the mother to each daughter spacecraft, will measure about a million km. Inside these spacecraft will float “freely falling” test masses – cubes with sides measuring abut 46 mm.
• Laser interferometers will accurately measure changes in the distance between these cubes. If they should be affected by a gravitational wave, the minute changes in this distance are measure by the interferometer.
• Hence option (d) is the correct answer.

Source: CSP2020