The intended meaning should be:
*She wrote the textbook for Spanish people who who currently do have to rely on translation from English*
(A) &(B) are redundant in meaning.
Analogy: I teach GMAT to people who want to learn GMAT.
She wrote a book in Spanish for people who speak in Spanish ----> not good.
(C) implies, she translated the book for people who do not need translation from English. This is polar opposite of the intended meaning. The actual meaning is--> these people need translation from English and that's why she wrote the translation.
(D) correct answer. Show cause and effect very clearly by using "so that". They need the translation so she wrote the book.
(E) implies, people were unable to speak Spanish untill she published the book. The book doesn't help people speak Spanish. For sure, this book is not for a traveler to learn Spanish

More on (A) & (B):
Deborah's intention was to free these people from the burden of relying on translations. So the two actions are not separate observation. We can not use "AND". We need cause and effect.
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