Explanation:

- Original sentence: Carolina opened a savings account to save money for later.

- Option 1: for a cold day

- This option doesn't fit as a common phrase. "Cold day" isn't typically used in this context.

- Option 2: for a rainy day

- This is a well-known idiom meaning saving for an unforeseen or difficult time in the future.

- Here's the correct substitute for "money for later."

- Option 3: for a sunny day

- A "sunny day" tends to imply a positive occasion. It isn't used for saving with precaution in mind.

- Option 4: for a hot day

- Similar to option 1, "hot day" isn't usually used in this context. It lacks connection to reserves or savings.