Final answer:
The eusporangiate fern in question is A. Whisk fern (Psilotum), known for its evolutionary loss of leaves and roots, and characterized by green branching stems with knob-shaped sporangia.
Step-by-step explanation:
The eusporangiate fern exhibiting evidence of leaf loss and represented by today's findings is A. Whisk fern (Psilotum). Whisk ferns, belonging to the class Psilotopsida, are characterized by their green stems with dichotomous branching, absence of true leaves and roots, and bear small yellow spherical sporangia at the tips or nodes of their branches.
This evolutionary reduction of structures is a consequence of natural selection which has led to the loss of characteristics that were no longer favorable in certain environments. Whisk ferns are interestingly linked to ferns through DNA analysis, suggesting a closer relation than previously thought. Evidence can be seen in Psilotum, as it demonstrates a profound example of eusporangiate development with its conspicuous green stems and knob-shaped sporangia, which is indicative of evolutionary adaptations over time.