💡 Detailed Explanation
The correct choice is 'がっ' (from '〜がる' - to show signs of / behave in a way that shows emotion). This suffix converts an I-adjective like '怖い' (scared) to a verb ('怖がる' - to behave fearfully / show fear), which is used to describe the feelings of a third person ('妹' - younger sister). Sentence translation: 'My younger sister is showing great fear of dogs.' 'らしくて' (looks like) and 'そうに' (seemingly) do not fit the raw stem transition perfectly here.