Which of the following items is only permitted in carry-on baggage?

The option mentioning a spare lithium battery is the correct choice because regulations governing air travel allow spare lithium batteries to be carried in the cabin as part of the carry-on baggage. Airlines and regulatory bodies have specific guidelines concerning lithium batteries due to their potential hazards. These guidelines specifically limit the amount and types of lithium batteries that can be transported in checked luggage due to risks associated with overheating or fire.

In contrast, a small clinical thermometer may have restrictions based on its contents (such as mercury), which could lead to it being categorized as restricted or prohibited if it poses a risk of breakage or leakage. The permeation device is typically subject to restrictions as it can contain hazardous materials. Checked ammunition is generally only permitted in checked baggage in compliance with specific regulations, as it poses a significant safety risk if carried in the cabin area.

Understanding these distinctions is essential for ensuring compliance with air travel safety regulations and maintaining safety during flights.