For at least two decades or more previous to when the Venetian artist Giovanni Battista Tiepolo died in 1770, he was the most admired painter in Italy.
A. For at least two decades or more previous to when the Venetian artist Giovanni Battista Tiepolo died in 1770, he was -- at least two decades or more is redundant
B. For at least two decades before the Venetian artist Giovanni Battista Tiepolo died in 1770, he had been - Correct
C. For at least two decades or more before the Venetian artist Giovanni Battista Tiepolo's dying in 1770, he had been -- same as A
D. When the Venetian artist Giovanni Battista Tiepolo died in 1770, for at least two decades previously he was - awkward
E. When the Venetian artist Giovanni Battista Tiepolo died in 1770, having been for at least two decades before that -- not even a sentence
"had __ed" refers to something earlier or completed. so, to refer to an EARLIER or DISCONTINUOUS interval, you'd use this tense.
consider:
• Ray had spent almost 20 years in prison before he died at the age of 90.
—> he could have served his sentence when he was much younger
—> he could have served 5 years at one time + 10 years at another time + 5 more years at another time
etc.
• Ray spent almost 20 years in prison before he died at the age of 90.
—> this means he spent his LAST twenty years incarcerated.
if you see a decision between PAST TENSE and "HAD ___ED"... make that decision ABSOLUTELY LAST.
if you can decide ANYTHING ELSE in the sentence, then ALWAYS make EVERY other possible decision before that one.
in fact, I'm not sure I can even recall seeing an official problem that actually REQUIRED a decision between "had ___ed" and the regular past tense.
Answer B