I replied: "The streets are beautiful"; which was graded wrong. The correct answer was: "The streets are pretty."

"Pretty" and "Beautiful" are a synonym, so both should be correct.

"ulice" is feminine in Czech? Doesn't looks like its neuter.

Why is it "hezké and not "hezcé" ?

so why did you reject "the streets are attractive" and "the streets are nice" as acceptable answers?

Is ulice both singular and plural?

I used "road" instead of "street" ("The roads are pretty") Is that really wrong? I could imagine a distinction like улица vs. дорога in Russian.

Why is the answer "These streets are pretty" not accepted? It is more of a demonstrative pronoun than an article.