Correct Answer: secure

Solution : The correct choice is the first option.

Explanation:
The antonym secure aptly conveys the opposite meaning, emphasising a state of safety and protection, which contrasts with the word vulnerable that means weak and easy to hurt physically or emotionally.

The meanings of the other options are as follows:

• Sensitive means easily affected, influenced, or harmed.
• Weak means lacking strength or power.
• Defenceless means without protection or the means to defend oneself.