Hi,

The Classical age of Greek culture ended with the defeat of Athens and Sparta; the (i) ______ effect of the long war was the weakening and (ii) ______ of the Greek spirit, and the (iii) ______ of the creativity of the prominent artists.

So for the first blank the clue is long war i.e. the war lasted for a long time so the effects should be in line with the same
A. immediate-Irrelevant-OUT
B. ironic- There is nothing ironic in the sentence- OUT
C. cumulative- Correct- As the war last for a long time its effect could be cumulative. Better than the other options

For second blank the clue is weakening something synonymous to weakening could work
D. rebuilding- Nope- OUT
E. corrosion- Correct.
F. separation- Clearly irrelevant

As for the third blank, I suppose curbing and stifling are synonymous. Can someone help me on this.
G. development
H. curbing
I. stifling

Thanks!!
Hope this helps!