Determine The Tense

After she had written three articles and edited the final proofs of her book, Rita (decides, decided, will decide, had decided, has decided) to take a break before beginning her next project.

The correct choice is "decided." The sentence tells you that Rita makes this decision after she had completed certain projects, so the actions exist in the past. "After" suggests that Rita's decision takes place in the past as well, though it happened after the other decisions. Therefore, "had decided" is wrong (then all of the actions would happen at the same time) and "decided" is correct.

The rules which govern the sequence of verb tenses are very complicated and too cumbersome to memorize. Rather, isolate the verbs in the sentence and the chronological markers like "after" or "during." Then use your ear to determine which option sounds best.