The correct answer is (a). Harisena was the court poet of Harshavardhana, the sixth-century emperor of the Indian subcontinent. He wrote a biography of Harsha, the Harshacharita, which is a valuable source of information about the time period.
Kalhan was a Kashmiri historian who wrote the Rajatarangini, a chronicle of the kings of Kashmir. The Rajatarangini is an important source of information about the history of Kashmir, but it does not contain much information about the time of Harsha.
Kalidasa was a Sanskrit poet and dramatist who lived in the Gupta period, which was about two centuries after the time of Harsha. Kalidasa’s works are not a reliable source of information about the time of Harsha.
Therefore, the correct answer is (a).