The fort that was known as the `Key of Deccan` is Asirgarh. Option 1, Kalinjar, is incorrect as it is a fort in Uttar Pradesh, not in the Deccan region. Option 2, Ajaygarh, is also incorrect as it is located in Madhya Pradesh, not in the Deccan. Option 4, Gulbarga, is incorrect as it is a city in Karnataka and does not refer to a specific fort. The correct answer is option 3, Asirgarh, as it is a strategically important fort situated on a hilltop in Madhya Pradesh. It was often referred to as the `Key of Deccan` due to its location and role in guarding the entrance to the Deccan region.