'Subsidiary Alliance' made which native state a "buffer state", giving…

The Subsidiary Alliance system (introduced by Lord Wellesley from 1798) let an Indian ruler keep internal administrative control and titular sovereignty while accepting a British Resident at court, disbanding most of the state's own army, and hosting a British-officered force whose upkeep the state itself paid — while foreign policy passed entirely into British hands. This let the Company project power and draw revenue without the cost of direct annexation, provided the ally's position was strategically useful.

Applied to India's geography, Awadh (Oudh) sat directly between the Company's core territory in Bengal and the still-unsettled northwest — the Maratha confederacy, the Rohilla chiefs, and (later) the Sikh kingdom in Punjab. Bringing Awadh under a subsidiary treaty (1801) let the British station troops there and control its external relations while leaving a friendly, semi-autonomous state as a shield in front of Bengal, absorbing the risk and cost of frontier defence without full administrative responsibility — precisely the 'buffer state' role the stem describes.

• Hyderabad was the very first state to sign a subsidiary alliance (1798), but that treaty was aimed at neutralising the Nizam's ties to French officers and containing Tipu Sultan's Mysore in the Deccan — a different strategic problem, not a northwest frontier buffer.

• Punjab remained an independent, powerful Sikh kingdom under Ranjit Singh through this period; it never entered the Subsidiary Alliance system at all and was annexed only decades later after the Anglo-Sikh Wars, so it cannot be the state this treaty framework turned into a buffer.

• Gwalior's Scindia rulers did accept a subsidiary alliance (from 1803/1817), but their strategic significance to the British lay in curbing a major Maratha military power, not in shielding Bengal's frontier.

So among the options, only Awadh fits both the geography and the documented British rationale — it is the native state the Subsidiary Alliance turned into a buffer, retained as a subordinate ally for decades (finally annexed only in 1856) precisely because of this frontier-shielding value.