The most appropriate definition of a natural resource is (b) a gift of nature which is very useful to mankind.
Explanation: A natural resource is something found in nature that is valuable to humans, like water, air, soil, minerals, forests, and animals.
Why other options are incorrect:
(a) present only on land:
This is not accurate as many natural resources, like air and water, are not solely found on land.
(c) a man-made substance placed in nature:
Natural resources are naturally occurring. Anything man-made is not considered a natural resource.
(d) available only in the forest:
While forests are a source of many natural resources, they are not the only source.