Select the most appropriate option to fill in the blank.

The ____ weapons which are easily available in the market are a big threat to the security of some nations.

deadly

The correct option to fill in the blank is deadly.

The sentence is about the threat posed by easily available weapons to the security of some nations. The word "deadly" is the most appropriate word to use in this context, as it refers to the ability to cause death.

The other options are not as appropriate:

• Heated means to be angry or upset. It would not be accurate to use "heated" in this context, as weapons are not angry or upset.
• Casual means not serious or important. It would not be accurate to use "casual" in this context, as weapons are serious and important.
• Valuable means worth a lot of money. It would not be accurate to use "valuable" in this context, as weapons are not valuable in the same way that a painting or a car is valuable.