Hi powermyscoreplz!
I would actually interpret that last sentence to mean there was likely plant life in the area at the time (as the vegetation is what allows the clay to build up and cause sandslides and the end of the sentence mentions how the Gobi area was a "field with
plant life and rain"), but regardless, the question posed in (C) is not whether or not plants existed at the time, but rather it is whether or not fossilized plants have ever been found in sandstone in the Gobi desert.
The passage does not answer this question, directly or indirectly, so (C) is incorrect. If plants did not exist in the area at the time, do we know if fossilized plants have
ever been found in the sandstone there? No! What about if there were plants in the area at the time (as the passage seems to suggest)? Still, we don't know anything about the fossilization of plants, nor do we know anything about if any fossilized plants from that time have ever been found! This leaves us uncertain about any possible answers to the question posed in (C). You can also keep the general context clues from the passage in mind, as the passage heavily deals with the fossilization of dinosaur, lizard, and mammals - not plants. This should make you less inclined to select answer choice (C) off the bat, and after a brief survey of the passage to confirm that there is no evidence to support an answer to (C), you can feel confident eliminating this answer!
I hope this helps
Kate