The discovery of eris

a. was not surprising, because other kuiper belt objects approaching the size of pluto had already been discovered
b. was not surprising, because the existence of a massive "planet x" had been predicted nearly a century ago
c. was surprising due to its "backwards" orbit around the sun
d. was surprising since we thought we knew about all large objects in the solar system

D

Explanation:

After discovery of Eris, initial calculations proposed Eris to be slightly larger than Pluto. Hence, NASA described Eris as the tenth planet of the Solar System, initially.

This initiated a debate in the scientific community, where major challenged could have been faced due to the possibilities of other similar size objects to be discovered in future. This led IAU (International Astronomical Union) to give a scientific definition to the word Planets and also led Pluto's disqualification as a planet, creating a new category of Dwarf Planets.