Select the most appropriate ANTONYM of the underlined word.

My niece is an amateur artist. I hope she becomes famous one day.

The correct answer is Expert.

To find the most appropriate antonym (opposite word) of the underlined word, let us first understand its meaning in the context of the sentence:
Amateur: A person who engages in a pursuit, study, science, or sport as a pastime rather than as a profession, or someone who is relatively inexperienced or unskilled in a particular activity.

Now, let us examine the meanings of the given options:
1. Boring: Not interesting; tedious.
2. Freelancing: Working for different companies rather than being permanently employed by one.
3. Expert: A person who has a comprehensive and authoritative knowledge of or skill in a particular area; a professional.
4. Decent: Conforming with generally accepted standards of respectable or moral behavior; satisfactory.

Since an amateur is a non-professional or beginner, the direct opposite is someone who has high skill or professional mastery, which is an expert. Therefore, Expert is the most appropriate antonym.