Which sentence is most clearly written?

The most clearly written sentence is the one that uses a colon to indicate that what follows is an elaboration or explanation of what was previously mentioned. In this case, the first sentence effectively signals that the action—the renewal of the library card—is a specific detail that illustrates the routine described. The use of the colon helps to create a clear separation between the introductory clause and the action being highlighted, enhancing clarity and comprehension.

In contrast, the second sentence employs a comma to connect the two ideas, which can create a less distinct separation between the main action and its elaboration. This can lead to a slight ambiguity in understanding that the renewal of the library card is a part of the routine described. By using the colon, the first sentence better organizes the information and presents it in a straightforward manner, making it the clearer choice.