We should _____for a rainy day. ?

The phrase “put something by” refers to the act of saving money or resources for future use, often for emergencies or unexpected situations, such as a rainy day. The expression "for a rainy day" specifically suggests preparing for times when funds might be low or when unexpected expenses arise.

Let’s analyze the other options:

A) "put down" does not fit the context, as it typically refers to placing something in a specific location or can imply criticism.
B) "put into" signifies placing something inside another thing and lacks the connotation of saving for future needs.
C) "put things off" means to delay or procrastinate, which does not relate to the idea of saving resources.

Given the context of financial prudence and preparation implied by "a rainy day," option C, "put something by," serves as the only fitting phrase. It encapsulates the notion of being wise and anticipating potential needs by setting aside resources in advance. Therefore, it is the most appropriate choice in this scenario.