The most likely outcome for the yucca plant if the yucca moth goes extinct would be:
The yucca plant would also go extinct.
This is because the yucca plant relies on the yucca moth for pollination, and without the moth, it would struggle to reproduce. While other responses might seem plausible, the specific coevolution between the two species means that the yucca plant has likely adapted to depend exclusively on the yucca moth for its pollination.