What are the key strategies to help reverse land degradation?

The question asks how to help reverse land degradation. Let's evaluate each statement:

• a) Implementing sustainable land management practices. Sustainable land management practices, such as crop rotation, conservation tillage, and agroforestry, are crucial for maintaining soil health, preventing erosion, and preserving biodiversity, thereby directly reversing or preventing land degradation. This is a correct strategy.
• b) Incorporating indigenous knowledge and perceptions. Indigenous communities often possess traditional ecological knowledge that includes sustainable land use practices developed over generations. Integrating this knowledge can provide effective, context-specific solutions for land restoration and management. This is a correct strategy.
• c) Restoration and rehabilitation of degraded lands. This involves active measures to repair damaged ecosystems, such as reforesting deforested areas, stabilizing eroded soils, and improving soil fertility. These actions directly aim to reverse degradation. This is a correct strategy.
• d) Restore freshwater ecosystems and plant more trees. Restoring freshwater ecosystems (e.g., wetlands, rivers) can improve water quality and availability, which are vital for healthy land. Planting more trees (reforestation or afforestation) helps prevent soil erosion, improve soil structure, enhance biodiversity, and sequester carbon, all contributing to reversing land degradation. This is a correct strategy.

All four statements (a, b, c, and d) describe effective strategies for reversing land degradation. Therefore, the correct combination includes all of them.

The final answer is $\boxed{\text{3}}$