The despised castes in Tibet were –

The answer is A) Smiths and butchers. In traditional Tibetan society, the hierarchy rested on occupation and purity rather than the varna scheme of the Indian subcontinent. Blacksmiths and butchers were regarded as impure because their work involved iron-working and killing animals, and they were kept at the margins of social and ritual life. Smiths in particular were often excluded from certain religious and social interactions.

Why the others fail:

• B and C include sweepers, a category tied to the Indian system of untouchability, not to the Tibetan pattern described in standard accounts of Tibetan society.
• C and D bring in shoemakers, who are not the groups usually named as despised in Tibetan sources.
• D keeps butchers but replaces smiths with shoemakers, so it fails on the same count.

Core concept: occupational purity and pollution as a basis of social stratification outside India. For Mains, this fits comparative discussions of caste-like hierarchies and how religious ideas of purity sustain social exclusion.

Trap: candidates import the Indian list of untouchable occupations such as sweepers and shoemakers and pick B, C or D. The question is about Tibet specifically, so the answer is the occupations that the Tibetan sources name.