Who among the following was given the additional charge as Governor of Tripura…

Under Article 160 of the Constitution of India, when a State's Governor is temporarily unable to discharge the functions of the office (for example, during leave, illness, or a vacancy before a new appointment), the President may direct the Governor of another State to hold the additional charge of that office until the substantive Governor resumes duty or a new Governor is appointed.

Tathagata Roy took charge as the Governor of Tripura in May 2015 and went on leave from 20 September 2015. On 24 September 2015, exercising the power under Article 160, President Pranab Mukherjee ordered that Keshari Nath Tripathi, who was already serving as the Governor of West Bengal since July 2014, would additionally discharge the functions of Governor of Tripura -- as recorded in the official press release issued by Rashtrapati Bhavan that day. Tripathi went on to hold this additional charge from 30 September until Roy resumed office on 20 November 2015.

Cross-check: this rules out the other three names for this specific assignment --

• Tathagata Roy was himself the Governor of Tripura who had gone on leave; the additional-charge order was made necessary by his own absence, not assigned to him.

• Krishan Kant Paul was serving as the Governor of Uttarakhand at this time; his own additional-charge assignment (of Manipur) had ended in May 2015, before this date, and never covered Tripura.

• Ram Nath Kovind was the Governor of Bihar at this time and had no gubernatorial charge, additional or substantive, relating to Tripura.

Result: Keshari Nath Tripathi was the person given the additional charge of Governor of Tripura by the order dated 24 September 2015; he went on to discharge that additional charge from 30 September until Roy resumed office on 20 November 2015.