Which of the following actions defines "extend"?

The action that defines "extend" is best captured by the notion of prolonging a deadline for payment. To extend something generally means to make it longer in time, distance, or range. In the context of deadlines, extending a deadline implies that you are increasing the amount of time allowed for completing a task or obligation. This aligns perfectly with the definition of "extend," which encompasses the idea of lengthening or enlarging something beyond its original limits.

The other options do not represent the action described by "extend." Reducing the size of something involves a decrease, which contradicts the concept of extension. Substituting an item for another implies replacing one thing with another rather than extending its duration or scope. Altering a process suggests making changes, which does not inherently relate to the act of extending in terms of time or space. Thus, the choice that refers to prolonging a deadline for payment accurately reflects the meaning of the term "extend."