Over the past few decades, despite periodic attempts to reign in spending, currencies in South America are devalued by rampant inflation
(A) are devalued Since the sentence is stating that something has been happening over a past few decades, we cannot use simple present 'are'
(B) are becoming more devalued Same as above. Simple present is used to show regular tasks or continuos tasks eg. A & B are beautiful towns etc.
(C) which have lost valueWhich always refers to the noun preceding it, in this case it would literally refer to south america. Hence illogical choice.
(D) have become devalued use of present perfect: have + past participle. Exactly whats required: a verb tense which shows an action started in past and whose effect continues till present.
(E) have since become devalued
Usage of since is wrong, seems like they have become devalued since past few decades, which is wrong
Hope this helps