Correct Answer: duel

Solution : The correct choice is the fourth option .

In the context of the sentence provided, duel is the most appropriate word to use. It describes a specific event where his father was involved in a fight with another person, resulting in his father's death when the boy was nine years old.

The meanings of the other words are as follows:

• Dwell : This verb means to live in a particular place, to reside, or to linger on a particular thought or subject.
• Drool : This verb means to let saliva run from the mouth, often as a result of excess saliva or due to certain conditions like being asleep or having seen something delicious.
• Dual : This adjective refers to something that has two parts, aspects, or components.

Therefore, the complete sentence would be: "His father was killed in a duel when the boy was nine years old."