Correct Answer: vague

Solution : The correct answer is the fourth option.

The antonym of the bracketed word definite is vague. Definite indicates that something is clear and well-defined whereas vague refers to something that lacks clarity and specificity and fits perfectly as the antonym of the given word.

The meanings of the other options are as follows:

• Monotonous describes something that is unvaried, resulting in boredom.
• Hazy means something that is unclear or has a lack of detail.
• Subtle refers to something that is not immediately obvious.

So, the correct sentence is: "There are vague weaknesses in their security arrangements."