In some large groups of plants, including dandelions, oaks, and willows, the biological species concept is complicated because the process of allows gene flow to occur between good that can be easily distinguished based on appearance.

A) hybridization; morphospecies
B) hybridization; ecological species
C) allopatric speciation; ring species
D) polyploidy; evolutionary species
E) polyploidy; ring species

The biological species concept is the concept which can define a species as an individual of a large population which can interbreed and produce an offspring in a population. The concept was given by Ernst Mayr.

Morphospecies refers to the species which can be characterised by the differences in their morphological features like in the Oak, willows and the dandelions.

The biological species concept is not valid or complicated in the case of the morphospecies like in the Oak, willows and the dandelions as the individuals of these three population can exchange their genes through the process of hybridization.

Thus, Option-A is the correct answer.