If I wanted to extend an order by 48 hours, what would I put?

The correct choice is to use "Extend by" when referring to increasing the duration of an order by a specified amount of time, such as 48 hours. The term "extend" conveys the action of lengthening or prolonging something in a clear and direct manner. In the context of an order, this phrase succinctly communicates the intention to add time to the existing timeline, making it easily understandable for all parties involved.

While other options might offer similar meanings in different contexts, they do not convey the specific contractual or administrative sense that "extend" does. "Lengthen" can suggest a more physical or tangible increase, while "modify" and "adjust" imply alterations that may not necessarily pertain to time specifically. Using "extend" ensures clarity and precision in the context of extending an order.