The correct option is A always erect
In a grassland ecosystem, the number of producers (mainly grasses) is always maximum, followed by decreasing numbers at second trophic level (herbivores), third trophic level (carnivores) and least number of apex predators.
Thus, a pyramid of numbers in grassland is erect.
Hence, the pyramid of number in a grassland ecosystem is always erect. So, the correct answer is 'always erect'.