The correct Active/Passive form of the given sentence is:

☀ The Deccan Plateau was ruled by the Maratha Empire.

Explanation for other options:

☀ The Deccan Plateau had been ruled by the Maratha Empire. This option uses the past perfect tense ("had been ruled"), which is not necessary in this context. The simple past tense is sufficient to convey the meaning.

☀ The Deccan Plateau was ruled over by the Maratha Empire. This option is grammatically correct, but it is not as concise as the correct option. The phrase "over" is not required and can be omitted.

☀ The Deccan Plateau has been rule by the Maratha Empire. This option contains a typo ("rule" instead of "ruled"). The correct word is "ruled." Additionally, the present perfect tense ("has been ruled") is not suitable for the context, as the sentence is discussing a historical event. The simple past tense is more appropriate.