A reminder should make the pending action easy to recognize. In a class-project example: "A quick reminder that we need your photo choice for the display by Thursday at noon. Please reply with the photo number from the shared list."
The task and response format are explicit. "Just checking in" may sound friendly, but the recipient still has to search earlier messages to discover what you need.
Use the current arrangement
Check the deadline before repeating it. If the group agreed an extension, use the new date. A reminder containing an old time can create more confusion than no reminder at all.
Add an appropriate route for difficulties: "If you cannot access the list, let me know and I will send the available choices." Do not offer help or flexibility you cannot provide. If you are simply asking about status, say that instead of implying the work is already overdue.
Practice two versions: a reminder sent before the deadline and a follow-up after it has passed. The second might say, "I have not received your choice yet. Could you let me know when you can send it?" Keep the tone proportionate to the relationship and task. Clear reminders reduce unnecessary searching and let the recipient either complete the action or explain the obstacle.