1. This girl is ... dancer in our group.

Step 1: Identify the sentence structure. It's a comparative sentence comparing the girl to others. Step 2: Analyze the options. 'good' is positive, 'better' is comparative, and 'the best' is superlative. Step 3: Determine the correct form. Since it's comparing to 'our group', the superlative 'the best' is appropriate.

c) the best