"Your cat is pretty."
From earlier notes on Duolingo: When we want to say that something is something, we use the word 是 shì, which is the verb “to be”. This is only when talking about two nouns or a pronoun and a noun. Remember to use 很 hěn when describing what something is like using an adjective.
When we want to describe something we don’t use the verb “to be”, we have to put something else before an adjective. The most common word used to do this is 很 hěn, which literally means very. It goes between the subject (who or what we are describing) and the adjective (what we are describing it as). Since 很 is used so often like this, it oftentimes doesn’t mean “very”, just a link between a noun and an adjective.
Throwing my hat in the ring: in China you would always say "hen3" in a situation like this. Its more of a sentence flow thing, than a grammatical thing from what I understand. I lived there for a year and the natural flow would be to include "hen3" in my experience.
That said, the literal translation would still be "Your cat is very pretty."