This question tests knowledge of comparative adjectives in English grammar. Comparative adjectives are used to compare two things. The way a comparative adjective is formed depends on the number of syllables in the adjective.
For most one-syllable adjectives, we add '-er' (e.g., tall -> taller). For adjectives ending in 'y' with two syllables, like "pretty," we change the 'y' to 'i' and add '-er'. For adjectives with three or more syllables, or some two-syllable adjectives not ending in 'y', we use "more" before the adjective (e.g., beautiful -> more beautiful).
In the case of "pretty," it is a two-syllable adjective ending in 'y'. Therefore, its correct comparative form is "prettier."
The correct sentence is "She is prettier than her sister." This sentence correctly applies the rule for forming the comparative of "pretty."
- "She is more pretty than her sister" is incorrect because "more" is not used with adjectives like "pretty" that form their comparative by adding '-er'.
- "She is more prettier than her sister" is grammatically incorrect. This is a double comparative, using both "more" and the '-er' suffix, which is redundant and never used in standard English.
- "None of these" is incorrect because option B is a grammatically correct sentence.