Which of these words a sense of purpose? a. contention b. intention

That would be B. Contention

Contention is a heated argument or the insertion into a heated argument while intention is the aim or plan or doing something for yourself benefit or others as it would be your intention

The purpose of beginning the selection with Mama's recollections and then moving to Dee's arrival is to emphasize the contrast between Maggie and Dee. It helps the reader get a better understanding of why Maggie and Dee are so different from Mama's perspective.