Which of the following is referred to as Aster?

The term "Aster" refers specifically to a genus within the family Asteraceae, known for its flowering plants including a wide range of species commonly cultivated for their beautiful, daisy-like blooms. Asters are typically characterized by their vibrant colors and blooming habits, which can provide a crucial source of late-season nectar for pollinators.

In contrast, Brunnera is known for its attractive foliage and small blue flowers, Anemone encompasses a genus recognized for its diverse flowering forms, and Astilbe is famous for its feathery flower spikes and fern-like foliage. Each of these plants has distinct characteristics and does not belong to the same genus as Aster, hence reinforcing why Aster is the correct choice in this context.