Final answer:
Endospores are specialized structures produced by certain bacteria that provide resistance to extreme temperatures and other harsh conditions, but these are not found in archaeans. Therefore, the correct option is 1.
Step-by-step explanation:
The structures produced by bacteria but not archaeans that are extremely resistant to boiling and freezing temperatures are endospores. Bacteria have the unique ability to form endospores when faced with adverse environmental conditions. Endospores are not a feature of archaeans, and they are the structures that enable certain bacteria to withstand extreme environmental stresses, including high temperatures, freezing, and dessication. Not to be confused with reproductive spores formed by fungi, endospores essentially protect the bacterial genome in a dormant state, allowing the bacteria to revive and become active again once favorable conditions return.