A) After gradual declension down
Declension & Down are the same thing. This is redundant. Hence, this option is incorrect.
(B) Following a gradual declension down
We still have redundancy error with declension and down.
(C) After gradual declining down
declining down - redundancy error.
(D) After gradually declining
Seems okay. Down is now removed, hence rectifying redundant error.
(E) Following gradually declining
Redundancy is resolved by removing down here. But, we have an error with "following"
Following can be used in 3 different ways
as a preposition (followed by a noun): Following months of uncertainty, the government has finally announced its decision.
as an adjective (only before a noun): She arrived the following day.
as a pronoun: Please read the following carefully.
In this sentence,
Following is used as a preposition. Following X, Y happened.
Gradually is modifying declining (this is adjective). Entire gradually declining is acting as an adjective modifying the noun - worksheet. As mentioned above, we should have a noun after "following'. For this reason, this option is incorrect.