"Selcen reads that."
Translation:Selcen onu okur.
That is correct. Duolingo gives some tips on certain lessons, but it also expects you to figure some things out for yourself. I advise using Duolingo in conjunction with a good basic grammar book or website. The "Turkish Grammar" page on Wikipedia is flawed, but it's one place to start. The section on "Pronouns" explains how they decline.
In English we usually say "subject" and "object" rather than nominative and accusative unless we're getting into the weeds with grammar, which, to learn another language correctly, you kind of have to do. Think of the difference between "he" and "him" and "I" and "me." You would naturally say "He sees me" or "I see him." "He" and "I" are nominative pronouns and "him" and "me" are accusative (and also dative, but that's another story). Turkish goes further and differentiates between subject and object pronouns for "it" and "this" and "that." "O" can be either he, she, or it as the subject: It is good. When it is both the direct object (accusative) and definite, it changes to "onu." "Bu" is this; "şu" and "o" are that. ("O" can also be it, he, or she, depending on context.) In English, "this" and "that" remain the same whether they are subject or object pronouns, but not in Turkish. If you think about it, we don't have to say "the this" or "the that" to indicate that they are definite pronouns. They are demonstrative pronouns and specific by nature. ("The" is only used before nouns, not pronouns.) That is why it is "Selcen onu okur" or "Selcen şunu okur." If the sentence to be translated were "Selcen reads this," it would be "Selcen bunu okur."