The first split:
Is/was: Was indicates a completed action in the past. The simple past verb tense is wrongfully applied here as its stated that the Mediterranean island is still ungovernable. This would imply that the effect of the distrust and neutralization is still active in the present. The simple past verb tense is incorrect. We can, therefore, eliminate C and E.
The second split:
A just sounds really weird, but we can't play by ear. So let's discern the grammar: We can discount A because of a parallelism error.
We can spot the parallelism marker of
and: X and Y. To distrust and neutralize. According to the principle of parallelism, comparable sentence parts must be structurally and logically similar. To distrust is a verb that takes the infinitive form so according to the principle of parallelism neutralize must do the same. So we can discount A as an answer option.
The third split:
C is definitely parallel in the sense that they both use progressive tenses. But let's delve a little deeper. Learned distrusting and neutralizing all written laws. The sentence sounds incomplete for some reason.
I might be wrong, but I think the fact that we mention
the reason idiomatically it should include the word the reason that. I assume this is an idiomatic error, so we discount answer choice C.
Hence we are left with D. D is correct because of the aforementioned reasons of other posters.
Hope this helped. Looking forward to your feedback!