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My answer is (D).It took me 01:16.

(A) Verb ("cost") is preferred to Noun ("a cost").
What is the antecedent of "which"? It should be "neglect" not "effects". So "are" does not agree in number.

(B) Notice "and" after the comma. In "X and Y", X and Y should be parallel.
"and already costing the restoration fund" cannot be parallel with "New leaks caused by the torrential rain have damaged the historic Henson House".
But if "and already costing the restoration fund" is parallel with "significant in compounding the effects of neglect", the comma is not needed. In this interpretation, the sentence's meaning is different from that of (A) or (D), but because we do not know what the author intends to say, so meaning change is not a strong reason to disqualify B.

(C) In this version, "already the restoration fund costs of more than $70,000" can only be understood as an appositive. It is a noun phrase, so it should be the appositive of "new leaks". Yet the two are not equivalent.

(D) What is the antecedent of "which"? It should be "neglect" not "effects". Why "cost" is okay here? Well, it is in simple past tense.

(E) In this version, "and already costs the restoration fund of" can only be parallel with "have damaged the historic Henson House", but then, because the subject (new leaks) is plural, "already costs" should be "already cost".

A) significantly compounding the effects of neglect, which already are a cost to the restoration fund of
are a cost isn't the right usage therefore out

B) significant in compounding the effects of neglect, and already costing the restoration fund
significant isn't the right usage therefore out

C) significantly compounding the effects of neglect, already the restoration fund costs of
alreasy shouldn't be left hanging , moreover not conveying the right meaning therefore out

D) significantly compounding the effects of neglect, which already cost the restoration fund
The meaning is absolutely perfect therefore let us hang on to it

E) significant in compounding the effects of neglect, and already costs the restoration fund of
Similar reasoning as B

Therefore IMO D

Correct answer : D

New leaks caused by the torrential rain have damaged the historic Henson House, significantly compounding the effects of neglect, which already are a cost to the restoration fund of more than $70,000.

Eliminate B, and E: Adjective can only modify noun, pronoun or noun phrase, it wrongly modify verb compounding.
Significantly is adverb, correct usage is with verb compounding

B) significant in compounding the effects of neglect, and already costing the restoration fund
E) significant in compounding the effects of neglect, and already costs the restoration fund of

Left with A, C and D
If we look at the bold wording below and relate to start of sentence "New leaks caused by the torrential rain have damaged the historic Henson House", the event is over, and its effect are cost to restoration

A) significantly compounding the effects of neglect, which already are a cost to the restoration fund of
Wrong:
- option A, says there is something other too have happened, and additional rain damage already cost to restoration, demeans the intended meaning of sentence,
plus preposition "of" must follow only noun, pronoun and gerunds

C) significantly compounding the effects of neglect, already the restoration fund costs of
Option C is little more messy, why could "the restoration" (singular) funds costs (plural) of more than, meaning of sentence creates ambiguity.

Option D - Corrct one
D) significantly compounding the effects of neglect, which already cost the restoration fund
- Adverb modify verb
- no meaning ambiguity
- SV Agreement done correct.

A) significantly compounding the effects of neglect, which already are a cost to the restoration fund of
which modifies 'the effects of neglect'(plural) and when rephrased you get "the effects of neglect are a cost to".
'a' cost is singular. Subject and verb does not agree. Eliminate.

B) significant in compounding the effects of neglect, and already costing the restoration fund
Only adverb can modify the verb. Adjective modifies noun/pronoun. Significant, an adjective, is modifying a verb, "compounding". eliminate.

C) significantly compounding the effects of neglect, already the restoration fund costs of
Noun phrase "already the restoration fund costs of" modifies the subject which is "New leaks" and does not describe the subject very well and looks out of place. Modifier reference error. Eliminate.

D) significantly compounding the effects of neglect, which already cost the restoration fund
which modifies 'the effects of neglect'(plural) and no grammatical error. Hence D is the correct answer choice.

E) significant in compounding the effects of neglect, and already costs the restoration fund of
Only adverb can modify the verb. Adjective modifies noun/pronoun. Significant, an adjective, is modifying a verb, "compounding". eliminate.

New leaks caused by the torrential rain have damaged the historic Henson House, significantly compounding the effects of neglect, which already are a cost to the restoration fund of more than $70,000.


A) significantly compounding the effects of neglect, which already are a cost to the restoration fund of Incorrect

which modifier sounds awkward

B) significant in compounding the effects of neglect, and already costing the restoration fund Incorrect

parallelism problem - .........damaged ............, significant in compounding........., and already costing...........

C) significantly compounding the effects of neglect, already the restoration fund costs of Incorrect

....., already the restoration fund costs of more than $70,000 - which affect restoration fund?

D) significantly compounding the effects of neglect, which already cost the restoration fund Correct

E) significant in compounding the effects of neglect, and already costs the restoration fund of Incorrect

parallelism problem - .........damaged ............, significant in compounding........., and already costs ...........

In option A, significantly, an adverb, is modifying the noun phrase compounding the effects, therefore all options with the same error could be eliminated. We are left with the options B and E now. In E, there is a subject verb agreement issue- costs doesn't match with leaks which leaves us with B. Though it sounds a little weird, IMO B.