(A) undermines the theory by showing that there must be some other cause for the contorted shapes of large animals’ fossils. Most mammals avoided angiosperms but many still have contorted fossils. So, something other than angiosperms must have caused those fossil positions.

This doesn’t weaken the argument because, even if angiosperms did provide nutrition, they were also poisonous. We also still don’t know that dinosaurs ate angiosperms in the first place.

By showing that some dinosaurs ate angiosperms, (C) slightly strengthens the argument. It doesn't challenge the author's theory or her assumptions that angiosperms were toxic enough to cause dinosaurs' extinction or that the contorted fossil positions were due to these plants.

This doesn’t change the fact that angiosperms do produce amino-acid-based alkaloids. So it does nothing to weaken the theory or to show that the author’s many assumptions are invalid.

We still don’t know that dinosaurs ever ate angiosperms, but (E) shows that angiosperm overdoses can cause death. So, if anything, this slightly strengthens the theory by confirming that angiosperms are poisonous enough to occasionally kill mammals.