a) gravitation has been the hardest force to unite with electroweak and strong nuclear interactions.
Since gravity has no force carrier boson which can be detected in order to comprehend the nature of gravity, and due to vast amount of difficulties, gravity is the most complex and mysterious force that, till this day, seems to have no direct correlation with the strong and the electroweak force. There are several mysteries such as the n-body problem which can't be explain or understood entirely, we just know that gravity is there.
b) is wrong since the proof of gravitational waves will bring even more questions to the physics society, just like how the remnant gravitational waves from the early universe were supposedly detected but were later classified as instrument error due to space dust and other mishaps.
c) wrong since we already classified gravity as one of the four fundamental forces of nature and if gravity was a symptom of inertia then that means the Earth is either a halo or a disk that accelerating upward at 9.8 m/s^2
d)wrong because we can't say for sure if we will ever create a UFT, and all of the four forces can act as an attractive force.
