The last great ruling dynasty of Magadha was

Option A: Sunga - The Sunga dynasty came after the Mauryas, but there were other dynasties after them in Magadha.
Option B: Kanva - The Kanva dynasty followed the Sungas, but it was relatively short-lived and not as impactful as other dynasties.
Option C: Kusana - The Kushan Empire was a powerful empire, but it was mainly focused in the northwest of the Indian subcontinent, not Magadha.
Option D: Gupta - The Gupta Empire was the last major and significant empire that rose from the region of Magadha.
Therefore, the correct answer is Option D: Gupta because it represents the final great ruling dynasty of Magadha.