This is Rio's fourth time as the chief minister of the Northeastern state.
Rio is heading the 'People's Democratic Alliance' government in partnership with the BJP in the state.
BJP's Y Patton was administered the oath as the Deputy Chief Minister by the governor.
Ten other ministers were also administered the oath of office. The ten included five from the BJP, three from the Nationalist Democratic Progresstive Party (NDPP), an independent and a JD(U) MLA.
The outgoing chief minister T R Zeliang was also present at the function.
Rio had staked claim to form the government on March 4 with the support of 32 legislators in the 60-member House -- 18 MLAs of NDPP, 12 of the BJP, one of the JD(U) and an independent MLA.
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