Topic: Degree of Comparison,

Best Answer Explained in Below-:

To understand why "most beautiful" is the superlative degree, it's essential to grasp the concept of degrees of comparison in English adjectives. Adjectives can be compared in three degrees: the positive, comparative, and superlative.

1. **Positive Degree**: This is the base form of the adjective, describing a quality without comparison. For example, "beautiful."

2. **Comparative Degree**: This form is used to compare two entities. Typically, for one-syllable adjectives, we add “-er” (e.g., "tall" becomes "taller"), and for longer adjectives, we use "more" before the adjective (e.g., "beautiful" becomes "more beautiful").

3. **Superlative Degree**: This form is used to compare three or more entities, highlighting the highest degree of the quality being described. For most adjectives of three or more syllables, we use "most" before the adjective (e.g., "beautiful" becomes "most beautiful").

In the options provided:
- A) "the beautiful" is not a correct form.
- B) "more beautiful" indicates a comparison between two items, not the highest degree.
- C) "most beautiful" correctly identifies the superlative degree, indicating something that stands out as the highest among others.
- D) "best beautiful" is not a standard grammatical structure.

Therefore, option C, "most beautiful," correctly represents the superlative degree, making it the best choice.