The relationship between species richness and area was observed by -

The correct answer is Option (4) → Alexander Von Humboldt

The species-area relationship says that species richness rises with the area explored, but only up to a limit. The German naturalist Alexander von Humboldt noticed this in the South American jungles.

(4) Alexander Von Humboldt — Correct. Humboldt explored the South American jungles and found that, within a region, species richness rises with the area explored, but only up to a limit. Plotted on a log scale this relation is a straight line, log S = log C + Z log A.

NCERT Extract:
Patterns of Biodiversity – “… (ii) Species-Area relationships: During his pioneering and extensive explorations in the wilderness of South American jungles, the great German naturalist and geographer Alexander von Humboldt observed that within a region species richness increased with increasing explored area, but only up to a limit.”