Final answer:
The prefix "retro-" signifies a movement back or a return to a previous state or condition, often indicating a backward movement or reliance on styles and trends from the past.
Step-by-step explanation:
The prefix "retro-" means c) Back, behind. It is used to form words that indicate an action or movement towards the back or a return to a past state or condition. For example, when we talk about "retro" fashion, we refer to styles that were popular in the past and have come back into trend. Retroactive is another example, where something takes effect from a date in the past. This prefix is quite versatile and is used in various contexts to signify a step back in time or position. It is derived from the Latin word "retro" which means backward or behind. The prefix is commonly used in English to indicate something related to the past or a backward movement.