The correct answer is D.  Cyclospora cayetanensis is variably acid-fast, and often absorbs/retains little or no stain.  This is the reason that hot safranin staining and UV fluorescence microscopy have been recommended as alternative methods to diagnose cyclosporiasis.  The other images seen are Cryptosporidium parvum oocysts (answer A), an Isospora belli oocyst (answer B), and an atypical artifact (answer C).