Ecological pyramids are graphical representations of relationships between organisms at different tropic levels. They depict the relationship of organisms at various levels and also their interactions.
The bottom of an ecological pyramid is the broadest. Its base forms the first level of the food chain, the producers.
Successively higher levels of food chains are formed by consumers, in order primary consumers, secondary consumers and tertiary consumers.
So, option A. " Base" is the correct answer.