Presently the East African country is focussed on
privatization of state-owned enterprises,
expansion of democracy and political freedom, and
de-escalation of interethnic conflicts and ousting corruption.
" , and" before de-escalation shows end of the list, and hence, end of the parallel element.
Thus, underline words should be in parallel while "ousting corruption" need not be. However, "ousting corruption" is still another issue East African country is focused ON. As a rule of grammar, the preposition should be repeated before ousting corruption to avoid any confusion.
Now let us analyse various options:
(A)
Privatization, expansion, and de-escalation are in parallel no preposition before ousting corruption(B)
same as (A)(C)
correct usage (D) Prefer Verb > Adjective > Noun , if everything else is similar in two options (here C and D). Prefer direct usage "ousting corruption" over noun form "on ousting of corruption" which is awkward.
Privatization, expansion, and de-escalation are in parallel usage "on ousting of corruption" is inferior to - "ousting corruption" (E)
Privatization, expanding , and de-escalation are not in parallel... and no preposition before ousting corruption-Sameer
Press kudos if it helped 
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But they say the correct answer is B though- C is wrong because "The ‘and’ after freedom is unparallel"