Step 1: Understanding Paleozoic Flora.
Gangamopteris is a genus of fossil plants that existed primarily during the Paleozoic era, particularly in the Carboniferous period. It is a notable member of the Paleozoic flora.
Step 2: Analyzing the Options.
- Williamsonia is associated with the Mesozoic era, not Paleozoic.
- Dicroidium is a Mesozoic genus.
- Gangamopteris is a genus found in the Paleozoic flora, especially in the Carboniferous.
- Nilssonia is a Mesozoic plant.
Thus, the correct answer is (C) Gangamopteris.
Final Answer:
\[
\boxed{\text{Gangamopteris}}
\]