Correct Answer: Energetic

Solution : The correct choice is the first option.

I n the sentence, the word vivacious is used to describe the lady's personality. The word "vivacious" means lively, animated, and full of energy. And energetic is the closest synonym that conveys the same sense of liveliness and high spirits associated with being vivacious.

The meanings of the other options are as follows:

• Fearless : This term means lacking fear or courage.
• Prosperous : This term relates to wealth and success.
• Friendly : This term describes a person who is pleasant and kind.

Therefore, the correct sentence would be: "The audience was mesmerised by the speech delivered by the energetic lady."