The most rapidly dwindling natural resource in the world is (b) forests.
Explanation: While water is crucial and can face depletion in certain regions, deforestation is occurring at an alarming rate globally, making forests the most rapidly diminishing natural resource according to various environmental studies.
Why other options are incorrect:
(a) Water:
Although water scarcity is a serious issue in many parts of the world, the overall global water supply is not dwindling as rapidly as forests are being depleted. Freshwater is a renewable resource, provided the water cycle is not disrupted. However, access to clean water is a major problem for many populations.
(c) Wind:
Wind is a renewable energy source. It is constantly generated by the movement of air and is not being depleted like fossil fuels or forests.
(d) Sunlight:
Sunlight is a renewable energy source from the sun. It is constantly available and will not run out unless the sun ceases to exist, which is not a foreseeable event.