The correct option is
D Receptive nature
Option C is the correct answer. The trait that appeals the most to Ethan, as suggested by it being mentioned last in the first paragraph, is her openness to the world around her: “She had an eye to see and an ear to hear: he could show her things and tell her things and taste the bliss of feeling that all he imparted left long reverberations and echoes he could wake at will”. Hence it is Mattie’s receptive nature that appeals most to Ethan.
Option A is incorrect because the passage suggests that Ethan does not actually view Mattie as particularly well suited to farm labour. When first seeing Mattie, Ethan thinks to himself, after “looking over her slight person,” that “she don’t look much on housework”.
Option B is incorrect because the passage suggests that Mattie’s youth is not what Ethan values most about Mattie. Although the passage does note that “the coming to his house of a bit of hopeful young life was like the lighting of a fire on a cold hearth”, the narrator goes on to note that “the girl was more than the bright serviceable creature [Ethan] had thought her”, indicating that Ethan values something more in Mattie than simply her vivacity.
Option D is incorrect because although Ethan acknowledges that Mattie “ain’t a fretter”, there is no evidence that Mattie’s freedom from worry is what Ethan values the most about Mattie. The first paragraph lists several positive traits that Mattie has, with the most emphasis being placed on her openness to the world around her.