Which term signifies a positive increase or growth happening naturally?

The term that signifies a positive increase or growth happening naturally is "accretion." In various contexts, such as science, economics, or geology, accretion refers to the gradual accumulation or growth of something, often involving the addition of new elements or layers. For example, in geology, accretion can describe the process by which sediment builds up and adds to a landmass, leading to its enlargement over time. This concept embodies the idea of positive growth, highlighting how elements can converge to form a larger whole.

The other terms do not reflect a natural positive increase. "Decrease" indicates a reduction in size or quantity; "erosion" describes the process of being worn away, typically by natural forces like water or wind, which contributes to loss rather than growth; and "decline" refers to a downward trend or a reduction in quality, quantity, or strength. Thus, accretion stands out as the term that accurately encapsulates the notion of natural positive increase or growth.