Final answer:
Direct microscopic examination of a wet preparation of vaginal discharge provides the simplest rapid diagnostic test for Trichomonas vaginalis.
Step-by-step explanation:
The direct microscopic examination of a wet preparation of vaginal discharge provides the simplest rapid diagnostic test for Trichomonas vaginalis, which is option C in the given question. By examining the discharge under a microscope, healthcare professionals can identify the characteristic movement of Trichomonas vaginalis, which aids in the diagnosis. This method is convenient and can provide a quick diagnosis, allowing for prompt treatment.