Step 1: Concept Identification: The query concerns the protagonist Charudatta's dual identity in the Sanskrit play \textit{Mrichchhakatika} ("The Little Clay Cart").
Step 2: Elaboration: Charudatta, the play's protagonist, is an exceptionally virtuous individual who has become impoverished due to his extreme generosity. His social standing is explicitly defined through two aspects:
His birthright identifies him as a Brahmana.
His former occupation was that of a prosperous merchant and leader of a trade guild or caravan, known as a Sarthavaha.
The other choices are incorrect:
Nishada designates a tribal group residing in forests.
Kshatriya denotes the warrior or ruling caste.
Mlechchha refers to an outsider or foreigner.
Step 3: Conclusion: Charudatta's identity is established as a Brahmana by birth and a Sarthavaha (merchant) by profession.