Of the four girls, Mary is the ____. ?

The question "Of the four girls, Mary is the ____." requires a comparative adjective to indicate how Mary relates to the other three girls in terms of appearance. The options provided represent different forms of the adjective "pretty," which describes attractiveness.

Option A, "prettier," is a comparative form but implies a comparison to only one other girl, not three, making it inappropriate in this context. Option C, "pretty," is a positive form and does not convey any comparative information. Option D, "far prettier," also uses the comparative form, but adds "far," which can imply a greater degree without a direct conclusion regarding the best among four.

Option B, "prettiest," is the superlative form of the adjective "pretty." This form indicates that Mary stands out as the most attractive among the four girls, which is precisely what the question suggests. Since the prompt specifies a group of four, using the superlative "prettiest" conclusively identifies Mary as the top choice in terms of beauty. Thus, "prettiest" correctly fulfills the requirement of the sentence, making it the appropriate answer.