Sentence Correction

For the first time in the modern era, non-Hispanic Whites are officially a minority in California, which amounts to a little less than half the population of the state, down from nearly three-quarters only a decade ago.

A. which amounts to a little less than half the population of the state, down from nearly three-quarters only a decade ago
B. which amounts to a little less than half the population of the state, down from a decade ago, when it was nearly three-quarters
C. and that amounts to a little less than half the population of the state, down from a decade ago, when they were nearly three-quarters
D. amounting to a little less than half the population of the state, down from nearly three-quarters a decade ago
E. amounting to a little less than half the population of the state, down from what it was a decade ago by nearly three-quarters

OA:D

In this question I could easily pick the OA but I have a doubt in D. What is the antecedent of the modifier (down from what it was a decade ago by nearly three-quarters). Is this modifier an absolute phrase or an appositive ? As this modifier seem to refer to "% of california's population that is white". Just want to understand the construction of this modifier ?

Thanks
Akshay