Final answer:
To complete the sentence, we need to find the appropriate degree of comparison of the adjective 'smart' to describe how the girl compares to her brother. She is smarter than her brother.
Step-by-step explanation:
Adjectives with two syllables can form the comparative either by adding -er or by preceeding the adjective with more. These adjectives form the superlative either by adding -est or by preceeding the adjective with most. In many cases, both forms are used, although one usage will be more common than the other.
To complete the sentence, we need to find the appropriate degree of comparison of the adjective 'smart' to describe how the girl compares to her brother. She is smarter than her brother. In this case, we are comparing the intelligence of the girl to her brother.
By using the comparative form of the adjective 'smart', we can clearly state that she has a higher level of intelligence than her brother. Comparative forms of adjectives are used when comparing two things or people. In this sentence, we are comparing the girl to her brother, indicating that she possesses a higher level of intelligence, making her 'smarter'.