Final answer:
The paradox of altruism is described in this situation where Travis helps Carlos without expecting anything in return, but Carlos reciprocates the favor.
Step-by-step explanation:
The situation described in the question is b. The paradox of altruism. Altruism is the selfless concern for the well-being of others. In this situation, Travis helps Carlos without expecting anything in return, but Carlos reciprocates the favor by helping Travis with his guitar playing. This creates a paradox because altruism is typically defined as selfless giving, yet in this case, both individuals benefit from the exchange.