Let's analyze each option to find the most suitable one to complete the sentence correctly:

A. have long decayed: This option uses the present perfect tense ("have decayed"). However, the sentence is describing an action that occurred before a specific point in the past (the operation). Therefore, the past perfect tense is more appropriate.

B. have long been decayed: This option uses the present perfect continuous tense ("have been decayed"). Similar to option A, it doesn't convey the correct past timing of the action.

C. have long being decayed: This option contains an incorrect verb form ("being"). The present continuous tense is not suitable here.

D. had long decayed: This option uses the past perfect tense ("had decayed"). This tense is appropriate for describing an action that occurred before a specific point in the past (the operation).

E. had been decayed: This option uses the past perfect continuous tense ("had been decayed"). While it conveys the correct past timing, the continuous aspect ("been decayed") is not necessary in this context.