Step 1: Recognizing fossil types.
Among the listed genera, only Glossopteris is a plant fossil. The others are marine invertebrate fossils.
Step 2: Comparing the options.
(A) Glossopteris: Seed fern from the Permian period; indicator of the Gondwana flora.
(B) Fenestella: Bryozoan fossil (marine animal).
(C) Productus: Brachiopod fossil.
(D) Cidaris: Echinoid (sea urchin) fossil.
Step 3: Conclusion.
Therefore, Glossopteris is the only plant fossil in the list.