The spindle-shaped pyramid of number is observed in certain ecosystems due to specific ecological factors. Let us explore the different ecosystems and understand where the spindle-shaped pyramid of number is observed.

Aquatic Ecosystem:
- The spindle-shaped pyramid of number can be observed in aquatic ecosystems such as lakes and ponds.
- This is because the producers in aquatic ecosystems are mostly phytoplankton, which are small in size and have a short lifespan.
- The consumers in aquatic ecosystems, such as small fish and zooplankton, are larger in size and have a longer lifespan.
- Hence, the number of producers is higher than the number of consumers in aquatic ecosystems, leading to a spindle-shaped pyramid of number.

Grassland Ecosystem:
- The pyramid of number in grassland ecosystems is generally upright, with a broad base of producers and a narrow apex of top predators.
- However, in certain grassland ecosystems, such as those with a short growing season or prone to frequent disturbances, the pyramid of number may be spindle-shaped.
- This is because the producers in such ecosystems may have a short lifespan and may not be able to sustain a large number of herbivores.

Forest Ecosystem:
- The pyramid of number in forest ecosystems is generally upright, with a broad base of producers and a narrow apex of top predators.
- However, in certain forest ecosystems, such as those with nutrient-poor soils or low light availability, the pyramid of number may be spindle-shaped.
- This is because the producers in such ecosystems may have a low growth rate and may not be able to sustain a large number of herbivores.

Desert Ecosystem:
- The pyramid of number in desert ecosystems is generally upright, with a broad base of producers and a narrow apex of top predators.
- However, in certain desert ecosystems, where water is scarce and the primary productivity is low, the pyramid of number may be spindle-shaped.
- This is because the producers in such ecosystems may have a low growth rate and may not be able to sustain a large number of herbivores.

In conclusion, the spindle-shaped pyramid of number is observed in certain ecosystems due to specific ecological factors such as short lifespan of producers or low primary productivity. It is important to understand the pyramid of number as it represents the energy flow in an ecosystem and helps in understanding the ecological relationships between different organisms.

C)Forest ecosystem.