Although the author himself declared them unpublishable

for this question, i will choose option D as the correct answer. the option explains the most concise and apt use of the contextuality of engaged in a prepositional phrase then by which is a preposition.
The agreement between the prepositions and the adverb above is a strength to this statement. Hence, This is the most correct

Now, let's have a critical look at other options and the reason why there are incorrect
In option A, there is a case of direct reportage of statement which does not confer any observable difference to the work at hand

In option B, the statement talks on the singularity of the main clause which contrast with the subject at hand. The use of it in place of they renders this option null and void. Hence, it is incorrect

For option C, the phrase 'Great modernist' refers to a person (singular) which is not in concord with the word they that is used in the later part of this option. It weakens the writer's argument and it's a NO. Therefore, option C is wrong!!!

Option E: The statement is archaic with the use of engaged which doesn't define the statement. It weakens the argument in that, the adjective 'great' qualifies the noun 'Modernist' which should be followed by an auxiliary verb is, can, will, should, have, has etc which herein, is not. This option is therefore wrong.