Correct Answer: Gentle

Solution : The most appropriate antonym is the second option.

Explanation: Vicious typically describes something aggressive, cruel, or having a harmful intent. Gentle is an appropriate antonym because it denotes qualities such as kindness, mildness, and a lack of aggression.

The meanings of the other options are as follows:

Influential: This term refers to having the capacity to exert influence or power.
Privileged: This term refers to having special rights or advantages.
Poor: It denotes a lack of wealth or resources.

So, the correct sentence would be: The dog, usually known for its vicious behaviour, surprised everyone by being gentle and friendly.