Answer:
Megaliths were large stones thought to be used to observe the sky by ancient astronomers.
Step-by-step explanation:
We can easily eliminate the first two options. They are indeed instruments used to observe astronomical movements, bodies, etc. However, they are simply not stones, so they are not related to the question.
We are left with megaliths and gastroliths, both words ending with the suffix -lith, which means they are both related to stones. "Gastro" refers to something related to the stomach, which is precisely the case here. A gastrolith is a stone found within the stomach of some animals. Finally, we are left with megaliths, large stones used to build monuments in ancient times which were likely used as very rudimentary astronomical observatories.