Final answer:
The most likely source of the pollen is a fern based on the scenario provided. Palynologists can determine the type of plants present or trace plant migration based on pollen fossils.
Step-by-step explanation:
The most likely source of the pollen based on the given scenario is fern.
Pollen grains have a resistant outer shell that can be preserved in the geologic record. Each plant species produces a different sized and shaped pollen grain. In this scenario, the presence of significant pollen from algae, moss, ferns, gymnosperms, puffballs, and mushrooms suggests that the most likely source of the pollen is a fern.
Palynologists, who study pollen, can determine which type of plants were present or trace the migration of plants as climate changes based on the analysis of pollen fossils.