Explanation:

We need a singular noun in the blank. ‘Conditions’ being plural will be incorrect. As a noun ‘Parallel’ means ‘a similarity or comparison’, ‘Marker’ means ‘a felt-tip pen with a broad tip’ and ‘Mystery’ means ‘something that is difficult or impossible to understand or explain’. None of them are appropriate for the blank. Only ‘Style’ is the appropriate noun which is aptly modified by the adjective ‘unique’