Select the option that can be used as a one-word substitute for the given group of words.
Vivacious and enthusiastic

The correct answer is Effervescent.

Let's analyze the given group of words: "Vivacious and enthusiastic". This phrase refers to someone or something that is lively, high-spirited, bubbly, and full of energy.

Let's look at the definitions of all the given options to find the correct one-word substitute:
1. Introverted: Shy, quiet, and preferring to spend time alone rather than with other people. This is the opposite of being vivacious and enthusiastic.
2. Effervescent: Literally, this refers to a liquid that is giving off bubbles (fizzy). Figuratively, when used to describe a person, it means vivacious, enthusiastic, lively, and sparkling with personality. Thus, it is the perfect one-word substitute.
3. Rowdy: Noisy, rough, and disorderly. While it involves energy, it has a negative connotation of being disruptive rather than positive enthusiasm.
4. Brazen: Bold and without shame. It describes someone who behaves in an open and shocking way without feeling embarrassed, which is different from being vivacious and enthusiastic.

Therefore, the word that best substitutes "Vivacious and enthusiastic" is Effervescent.