This question tests verb tense , modifier concept and meaning
Evidence of the relatively small, icy object, estimated at no more than half a mile in diameter, was spotted by the National Aeronautics and Space Administration's robot spacecraft Cassini that has been orbiting Saturn since 2004.
a. at no more than half a mile in diameter, was spotted by the National Aeronautics and Space Administration's robot spacecraft Cassini that has been =
verb tense has been correct , but use of estimated at wrong meaning change - evidence is orbiting
b. as being less than half a mile in diameter, was spotted by the National Aeronautics and Space Administration's robot spacecraft Cassini that has been =
verb tense has been correct , but use of estimated as being wrong meaning change - evidence is orbiting
c. that it is no more than half a mile in diameter, was spotted by the National Aeronautics and Space Administration's robot spacecraft Cassini, which has been
verb tense has been correct , but use of estimated that it wrong , no requirement of it meaning change - evidence is orbiting
d. to be no more than half a mile in diameter, was spotted by the National Aeronautics and Space Administration's robot spacecraft Cassini,
No present perfect tense , hence wrong
e. to be no more than half a mile in diameter, was spotted by the National Aeronautics and Space Administration's robot spacecraft Cassini, which has been
Use of Which correctly modifies Cassini , meaning is intact - Cassini is orbiting , correct verb tense