Translation:Spring is the prettiest.
"Spring is the prettiest"???? Prettiest what? Come on Duolingo can you even speak English?
Native English speaker here. "Spring is the prettiest." is perfectly normal and natural English.
Don't forget, it comes from context. -"Which season do you like the most?" -"Spring is the prettiest"
I dont understand the convention for the superlative (e.g. He was the fastEST runner) . Does it just go without saying?
Was this translated by my 2nd language students? Where is the object in this sentence?
"Is" is not a transitive verb, it's a copula. The copula takes a compliment. A compliment is not only an object. "I am hungry" is exactly the same, no object, just an adjective as compliment.
I think that this sentence is grammatically correct, it just sounds weird.
I'm a native English speaker and I find it to be grammatically correct and normal sounding.