Which of the following evidence suggests that the Sahara Desert was once a lush green plain?

The correct option is

Cave paintings depicting rivers and animals

Explanation

The African Humid Period (also known as the Green Sahara), a phase during the Holocene epoch (approximately 11,000 to 5,000 years ago) when the Sahara region supported a savanna ecosystem with lakes, rivers, and abundant vegetation.

Statement-wise Analysis:

• Discovery of oil reserves beneath the sand is Incorrect. Oil reserves are formed from organic matter (primarily marine microorganisms) buried under high pressure and heat over millions of years. Their presence indicates ancient geological processes, not necessarily a "lush green plain" during the recent human past.
• Cave paintings depicting rivers and animals is Correct. Rock art and cave paintings found in locations such as Tassili n'Ajjer (Algeria) depict riverine animals (hippopotamuses, crocodiles), large savanna mammals (elephants, giraffes), and scenes of cattle herding and swimming. These paintings provide direct anthropological and ecological evidence that the region was once a fertile grassland capable of supporting diverse life.
• Presence of underground aquifers feeding oases is Incorrect. While the presence of fossil water in underground aquifers (such as the Nubian Sandstone Aquifer System) proves that the region once had significant precipitation and groundwater recharge, the specific evidence illustrating a "lush green plain" with visible flora and fauna is the rock art. Aquifers are geological features, whereas the paintings document the surface environment.
• Historical records from the Roman Empire is Incorrect. The Green Sahara period ended around 5,000 years ago due to shifts in the Earth's orbital parameters. The Roman Empire rose much later (after 27 BCE). By the Roman period, the Sahara had already desiccated into a desert, although the Romans did cultivate the fertile coastal strips of North Africa.

Key Takeaway:

The Tassili n'Ajjer cave paintings serve as the most vivid historical record of the Holocene Wet Phase, proving that the Sahara was once a humid savanna teeming with wildlife before desertification occurred.