Final answer:
Sentence C, which states that chess teaches students to think ahead about the consequences of their actions, best supports the student's argument about why chess is beneficial for elementary school kids.
Step-by-step explanation:
The student's essay argues that chess is a valuable activity for elementary school kids that teaches patience, strategy, and the importance of sacrifice. To support this argument with a relevant detail, sentence C would be the best choice. It states, "Chess teaches students to think ahead about the consequences of their actions." This is directly related to the author's point about chess promoting strategic thinking and future planning, which are cognitive skills beneficial not only in the game but also in real life. Indeed, as noted in other contexts, cognitive activities like playing chess are known for keeping mental faculties sharp. This aligns with the argument the student is making about the value of chess in the development of children's skills.