Correct Answer: deadly

Solution : The correct choice is the second option.

Explanation : Deadly means causing or being able to cause death or serious harm. In the context of discussing weapons and their threat to national security, deadly best describes the potential harm or danger they can cause.

The meanings of the other options are as follows:

• Heated means having a high temperature or showing strong emotion.
• Casual refers to something relaxed, informal, or not planned.
• Valuable means something of great worth or importance.

Therefore, the complete sentence would be: "The deadly weapons which are easily available in the market are a big threat to the security of some nations."