Explanation
- A
To detect neutrinos
Neutrino detectors are underground or ice/water Cherenkov experiments (INO, IceCube, Super-Kamiokande), not eLISA. This option is not the key.
- B
To detect gravitational waves
eLISA (evolved Laser Interferometer Space Antenna, now LISA) is a planned ESA space interferometer to detect gravitational waves at frequencies that ground detectors miss. Laser arms in space measure tiny spacetime strains. That is why (b) is the official key.
- C
To detect the effectiveness of missile defence system
Missile-defence effectiveness is tested by radars and interceptors, not by a space gravitational-wave antenna. This option is therefore wrong.
- D
To study the effect of solar flares on our communication systems
Solar-flare effects on communications are a space-weather topic (ionosphere, GPS). eLISA is not built for that. This option is not the key.
Summary. Official key is (b) To detect gravitational waves. LISA/eLISA places laser interferometers in solar orbit to hear millihertz gravitational waves from massive black holes and compact binaries. Neutrinos, missile defence and solar flares are different experiments. Ground detectors (LIGO/Virgo) already saw high-frequency waves; eLISA targets a complementary band.
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