The Subsidiary Alliance was a diplomatic strategy introduced by Lord Wellesley in 1798 to bring Indian princely states under British control without direct annexation.
A key feature of this alliance was that the British would station their troops in the territory of the allied Indian state to protect it from external threats and internal disturbances. In return, the ruler had to pay for the maintenance of these British troops and could not enter into alliances or wars without British permission.
This arrangement significantly curtailed the sovereignty of Indian rulers and expanded British political influence. Therefore, the correct answer is (D) Protection of native territory by British, as that was the promise used to justify the presence of British forces.