I don't think "Peter will eat this evening" is worth being counted as a bad translation for this sentence
I agree. I also put this and was marked wrong.
That was my first attempt too - but I think 'this evening' would actually be इस शाम को।
Maybe it's not oblique because ko is attached to evening. If it were a sentence where ko was attached to Peter, maybe that's when it switches to oblique?
What do you want to be oblique? Nouns and adjectives have cases; there are no adjectives here, पीटर as a proper noun will always be पीटर, and शाम is शाम in all cases in the singular.