Final answer:
Protecting mangroves has the highest benefit-cost ratio due to their role in coastal protection, flood mitigation, and ecosystem services which are cost-effective compared to artificial barriers, aiding in the sustainability of smallholder farms.
Step-by-step explanation:
Of the adaptation measures listed, protecting mangroves seems to have the highest economic benefit-cost ratio. Mangroves play a crucial role in coastal protection, reducing the impact of waves and storm surges, which can lead to substantial economic savings in the event of natural disasters like tsunamis.
Additionally, they provide essential ecosystem services such as hydrologic regulation by intercepting rainfall, promoting soil development, and potentially reducing flooding. Protecting and restoring mangrove forests are more cost-effective than constructing artificial barriers and promote biodiversity; they are also critical for the health and sustainability of smallholder farms, which are vital for food security and poverty reduction.
Agroforestry systems, which incorporate trees with crops and livestock, can also enhance the resilience of agriculture to climate risks, especially in regions vulnerable to climate change such as Sub-Saharan Africa.