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
B. For at least two decades before the Venetian artist Giovanni Battista Tiepolo died in 1770, he had been
C. For at least two decades or more before the Venetian artist Giovanni Battista Tiepolo's dying in 1770, he had been
D. When the Venetian artist Giovanni Battista Tiepolo died in 1770, for at least two decades previously he was
E. When the Venetian artist Giovanni Battista Tiepolo died in 1770, having been for at least two decades before that
A. For at least two decades or more previous to when the Venetian artist Giovanni Battista Tiepolo died in 1770, he was
B. For at least two decades before the Venetian artist Giovanni Battista Tiepolo died in 1770, he had been
C. For at least two decades or more before the Venetian artist Giovanni Battista Tiepolo's dying in 1770, he had been
D. When the Venetian artist Giovanni Battista Tiepolo died in 1770, for at least two decades previously he was
E. When the Venetian artist Giovanni Battista Tiepolo died in 1770, having been for at least two decades before that
This is a clear case of Past Perfect: one action happend in the past for a long period of time ("he had been the most admired"), before another action ("died").
That's why B is correct.
We can find in the other choices verb-errors, awkward structures or grammatical errors.