Answer:
Carl will do things for purely selfish reasons, but sometimes he is capable of great generosity.
Step-by-step explanation:
Frivolous means something of little importance, or trivial/superficial
Empathetic means being able to understand and share the feelings of others
Selfish means lacking consideration for other people; concerned chiefly with one's own personal profit or pleasure.
Juvenile means relating to a young person who is not yet old enough to be considered an adult
In this context, the most suitable word would be suitable. This can be inferred from the conjunction "but", which indicates a contrast between terms, and thus the opposite of generosity, or selflessness (giving to others), would be selfishness (keeping to oneself).
Hence: Carl will do things for purely selfish reasons, but sometimes he is capable of great generosity.