Final answer:
The sentence requires a comparative adjective that indicates why she preferred the blue dress over the green one. The exact word depends on the context, but could be 'prettier', 'cheaper', 'more sophisticated', or 'more suitable'.
Step-by-step explanation:
The blank space in the sentence can be filled with a wide variety of adjectives that compare two things, based on the context. For example, the word can be 'prettier', 'cheaper', 'more sophisticated' or 'more suitable' depending on what the basis of comparison is.
In the absence of a specific context, the comparative form of most adjectives could fit in the blank space, as long as it denotes something that influences the person's choice concerning the dresses.
Learn more about Comparative Adjectives