A synonym refers to a word or verb that has an actual or very similar aspiring to another word. it's employed in a sentence to avoid over and overuse of the identical word.
‘Obscure’ is an adjective that we use to discuss with something uncertain, unclear, or not fully known.
Example: The film was directed by an obscure director.
Here, option d contains an analogous meaning. Therefore, ‘unknown’ is that the right synonym for obscure.
Option a is wrong. The word ‘famous describes something well-known to several people. because it gives us a contrary meaning, we rule out this feature.

In option b too, we get a meaning almost like ‘famous’. Hence, option b is additionally proved wrong.
‘Rampant’ in option c could be a term relating something flourishing in an uncontrollable manner. This evidently holds no regard to ‘obscure’.

Note : don't confuse synonyms with antonyms. during a similar-looking set of options as given here, ensure you are doing not choose the alternative meaning words.