"dilatory" confused me, which means to prolong/extend something/make something slow. Direct opposite of productivity.

First blank:
The sentence says "composing more than 20 symphonies in his short lifetime" $\(\rightarrow\)$ that means he was very productive.
- abortive means unsuccessful or cut short $\(\rightarrow\)$ doesn't fit.
- prolific means highly productive $\(\rightarrow\)$ fits perfectly.
- professional could fit loosely, but "prolific" is the direct match for producing a large volume of work.

So Blank (i) = B. prolific.

Second blank:
"Despite this tremendous productivity, he was almost unimaginably ______ about even the most ... details" $\(\rightarrow\)$ The contrast suggests that even though he composed a lot, he was extremely careful/meticulous/perfectionistic.
- dilatory means slow/tardy $\(\rightarrow\)$ not fitting here (productivity shows he wasn't slow overall).
- perfectionistic $\(\rightarrow\)$ fits the idea of being extremely careful about details.
- filial $\(\rightarrow\)$ relating to a son/daughter, irrelevant.

So Blank (ii) = E. perfectionistic.

Third blank:
He was perfectionistic about even the most ________ details.
- minute (adjective) $\(\rightarrow\)$ means very small, fine details $\rightarrow$ fits "perfectionistic."
- paramount $\\(rightarrow\)$ means most important $\rightarrow$ but "even the most paramount" doesn't match "even" (which implies surprising for trivial things).
- penultimate $\(\rightarrow\)$ means second to last $\rightarrow$ irrelevant.

So Blank (iii) = G. minute.

Final answer:
B, E, G