The correct answer is option 1: upright.

A biomass pyramid represents the amount of biomass or energy present at each trophic level in an ecosystem. In a grassland ecosystem, the primary producers (grasses) have the highest biomass as they are able to access sunlight efficiently. This means that the biomass decreases as we move from the producer level to the herbivores and then to the carnivores.

In an upright biomass pyramid, the biomass decreases at each higher trophic level, which is the case in a grassland ecosystem. The primary producers have the highest biomass, followed by the herbivores, and then the carnivores.

Hence, in a grassland ecosystem, the biomass pyramid is upright, indicating a decrease in biomass at each higher trophic level.