prep SC:For at least two decades or more previous to when

[RonPurewal]

these tenses could both work in a context like this one.

"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.

both interpretations could work here, too.

[RonPurewal]

more generally—
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.

__

in this problem, for instance, both A and C have "at least" + "or more". that's a redundant combination, so, easy elimination right there.

once those two choices are gone, the verb tenses are a non-issue.