Final answer:
Rex is using a simile when he compares Jeannette to a mountain goat. Having this nickname may help Jeannette feel confident and capable.
Step-by-step explanation:
Rex is using a literary device called a simile when he tells Jeannette that she is "sure footed as a mountain goat." A simile is a figure of speech that compares two things using the words "like" or "as." In this case, Rex is comparing Jeannette's sure-footedness to that of a mountain goat.
Having this nickname can be seen as a compliment and may help Jeannette feel confident and capable. The comparison to a mountain goat suggests that she is agile, sure, and able to navigate difficult terrains with ease, which could boost her self-esteem and make her more confident in her abilities.