Explanation:

Let's analyze the given words by arranging their letters alphabetically.

- BOY: Arranged alphabetically becomes BOY. The word "boy" is meaningful.

- CAT: Arranged alphabetically becomes ACT. "Act" is a meaningful word.

- RAT: Arranged alphabetically becomes ART. "Art" is meaningful.

- MAT: Arranged alphabetically becomes AMT. This doesn't form a meaningful word.

- SIT: Arranged alphabetically becomes IST. This doesn't form a meaningful word.

- PEN: Arranged alphabetically becomes ENP. This doesn't form a meaningful word.

- Now, counting the meaningful words: Boy, Act, Art

- In total, we have three meaningful words.

Therefore, the correct answer is:

Option: 3, Three