The answer is C decided.
Determined synonyms:
Resolute, full of determination, purposeful, purposive, resolved, decided, adamant, single-minded, firm, unswerving, unswervable, unwavering, undaunted, fixed, set.
A. Youthful
B and D is considered negative while C means they have had very little experience on something
Because when you are reading a plot of a story you need to imagine yourself as the character or object and use your five senses.
here you go, hope this helps