Although the contingent of self-described experts who are in attendance at the International Symposium on Unexplained Phenomena in Deep Space Studies have written an open letter stating that the newly discovered interstellar object is an alien space probe, most astronomers who have studied the object closely agree that it is a comet.
Split 1# contingent is singular .. usage of HAS is correct
Eliminate A and B
Split 2 # 2 dependent clauses - comma splice ..
although…. But … A.
have written an open letter stating that the newly discovered interstellar object is an alien space probe, most astronomers
B.
have written an open letter stating that the interstellar object, newly discovered, is an alien space probe, most astronomers
C. has written an open letter stating that the newly discovered interstellar object is an alien space probe,
but most astronomers
D.
has written an open letter stating that
the discovery of a new interstellar object
is an alien space probe, most astronomers
illogical statementE.
has written an open letter stating that the
newly discovered interstellar object is an alien space probe, most astronomers
correctAnswer E