Alexander von Humboldt observed the relationship shown in the graph b…

The correct answer is Option (4) → Species richness

Alexander von Humboldt explored the South American jungles. He saw that within a region, species richness increased with increasing explored area, but only up to a limit. For many groups of organisms the curve is a rectangular hyperbola, given by S = CA^Z. On a logarithmic scale it becomes a straight line, log S = log C + Z log A, where S is species richness and A is area. The quantity on the y-axis is therefore species richness.

Option (1) Frugivorous (fruit-eating) birds — Incorrect. Frugivorous (fruit-eating) birds are a group of animals, not a quantity on an axis. The book uses them only as an example of very large areas (whole continents), where the slope of the line is steep.

Option (2) Biomass — Incorrect. Biomass is the mass of living matter. The species-area graph counts the number of species, not their mass.

Option (3) Extinction — Incorrect. Extinction is the loss of species. Humboldt’s graph shows how many species are found as area increases, not how many are lost.

Option (4) Species richness — Correct. In both the curve and the log line, S stands for species richness, and it is plotted against area.

Note: Half this question lives inside a graph. Give NCERT figures and their labels the same attention as the text, because NTA happily lifts questions straight off a curve.

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. In fact, the relation between species richness and area for a wide variety of taxa (angiosperm plants, birds, bats, freshwater fishes) turns out to be a rectangular hyperbola (Figure 13.2). On a logarithmic scale, the relationship is a straight line described by the equation
log S = log C + Z log A
where
S = Species richness A = Area
Z = slope of the line (regression coefficient)
C = Y-intercept”

Figure 13.2 Showing species area relationship. Note that on log scale the relationship becomes linear