Submit photos together, chat logs, call records, travel itineraries, engagement proof, letters from family and friends, and any evidence showing your ongoing relationship and intent to marry.

You should submit photos together, chat logs, travel records, engagement proof, and letters from family or friends.

I think Raj and Isaak already gave good answers. They just wanna see proof that you guys are in a genuine relationship.

I suggest, when applying for a K-1 visa, it's essential to provide strong evidence to prove like submitting pictures together or chat logs maybe.