"Tengo una fresa más roja que esa."
Translation:I have a strawberry that is redder than that one.
Este, esto, esta are terms used for 'this'.
Ese, eso, esa are terms used for 'that'.
You can find out more information by a simple google search.
This "ésa" is a pronoun - "... more red than that [one]." The accent mark may not be necessary, but it is not wrong, and in fact it helps to distinguish intent.
Duolingo is wrong here. Their record with accentuating demonstrative pronouns is spotty at best. Sometimes, as here, they chide you for accenting letters where it's right; other times, they say the accent is required. They need to clean up their act.
The article before 'strawberry' needs to be 'a' not 'the'. Be careful to keep them consist when moving their position in the sentence. You made it to mean only one strawberry is in possession rather than out of the multiple strawberries 'I have' one is redder than that [one].
I've heard both, depending on the speaker and the situation. "He was so embarrassed that his face just got redder and redder ..." That sounds OK to me. But so does " ... got more and more red ..." For this example exercise, "I have a strawberry that is more red than that one" doesn't sound as good to me as "redder". Anyway, my point is that I think both should be accepted because I believe that native English speakers say and hear both in common usage.