Executives and federal officials say that the use of crack and cocaine is growing rapidly among workers,
significantly compounding the effects of drug and alcohol abuse, which already are a cost to business of
more than $100 billion a year.

(A) significantly compounding the effects of drug and alcohol abuse, which already are a cost to business of

(B) significantly compounding the effects of drug and alcohol abuse, which already cost business

(C) significantly compounding the effects of drug and alcohol abuse, already with business costs of

(D) significant in compounding the effects of drug and alcohol abuse, and already costing business

(E) significant in compounding the effects of drug and alcohol abuse, and already costs business

As per OG explanation , option (B) is the correct answer.

Can I take a note from this example that "which" doesn't always refer to antecedent right before "," ( comma ) separator.
In this case, "which" is referring to "the effects" and "drug and alcohol abuse" is just an object of prepostion "of". So, if there exists an object of prepostion right before "," and a modifier starts with "which" after comma separator, I need to look at the previous clause carefully to determine the exact noun that is modified by "which" . Object of prepostion will not work in this case.

Please suggest. I have wrongly judged a number of questions of this type.

You are correct: when you have "X of Y" before a modifier of this type, the modifier could potentially refer either to "Y" by itself or to the whole unit "X of Y". It's up to context to determine which.

Thank you Ron !!