"Churches are religious buildings."
Translation:Les églises sont des bâtiments religieux.
As noted below, it is "les" églises because it is a general statement (in English we don't require definite articles for plural nouns, but we do for singular nouns "Snakes are dangerous animals/The snake is a dangerous animal").
To use "les", you would already have to be talking about specific buildings, but this sentence is completely out of context.
"Des" here is NOT "de + les", but the plural of "un".
That's correct. If you were marked wrong you must have written something different.
Assuming what you wrote was "Des églises sont les bâtiments religieux". The translation of that would be something like "Some of the churches are the religious buildings" which of course makes absolutely no sense. The reason it's "LES églises" is because we're talking about all churches as a concept.