"That is a sandwich."
Translation:Dat is een boterham.
You've been asked to translate "dat" which means "that". In English, It is a sandwich, you'd translate it like this: Het is een boterham.
Good question! I'm a native Dutch speaker. Dat and Die both mean that. A native speaker would automatically know when to use either, but it's not so easy to explain. In this sentence, Dat is used for emphasis, i.e. THAT is a sandwich. You would not say Die is een boterham. You could say Die boterham is lekker, or That sandwich is (tastes) good. If you were to say That is a man, in Dutch you'd say Dat is een man. But That man is crazy would be Die man is gek. I'm not sure how to explain this grammatically, but I hope you get what I mean.
I'm just a learner, but I believe the essential difference is that "dat is..." is followed by a noun (e.g. dat is een boterham : that is a sandwich), whereas "die is..." is followed by an adjective (e.g. die is duur : that (one) is expensive).
Een boterham is literally a slice of bread, but is often used to mean a sandwich. Een broodje literally means a small loaf but generally refers to a filled sandwich.
Picture of a boterham: https://upload.wikimedia.org/wikipedia/commons/1/1c/2014_Boterham_oude_kaas.jpg
Picture of a broodje: http://natuurlijkbijkaat.nl/wordpress/wp-content/uploads/2015/08/broodje-kaas.jpg