(A) has done seriously and possibly irreversible damage to
=> "seriously" is an adverb, so it can't modify the noun "damage"
(B) did damage that is seriously and possibly irreversible to
=> adverb "seriously" can't follow "is"
(C) damaged, serious and possibly irreversibly
=> the comma makes no sense here
=> the whole part ", serious and possibly irreversibly" is severely grammartically incorrect
(D) has done serious and possibly irreversible damage to
=> correct: "serious and possibly irreversible" modifies "damage"
(E) did damage, serious and possibly irreversible
=> "serious and possibly irreversible" seems to modify "horses" and this changes the intended meaning
=> even it is the case, E would be a run-on sentence
_________________

Correct answer: D

A: Not parallel/wrong adverb modifier "seriously" to modify noun "damage" (should be written "serious ... and possibly irreversible damage")
B: Wordy/awkward. Verb form (simple past vs. present perfect). "Seriously" should be the adjective "serious" as it modifies "damage."
C: Verb form as in B. "Serious" here should be an adverb ("seriously") because it modifies "damage." The clause "serious and possibly irreversibly" should end with a comma as it is separated off from the rest of the sentence.
D: Correct
E: "Serious and possibly irreversible" should be written as an adverb ("seriously and possibly irreversibly") and end with a comma.
_________________