Past-perfect tense

Consider the following question:

Monica had dinner at home. She ______ dinner when she ______ home.

(a) ate, came

(b) had eaten, had come

(c) had eaten, came

(d) ate, had come

According to me answer to the above question should be option (d), as first she came home and then she ate dinner, using present perfect. But the correct answer is (a). How is it possible, as using the option (a), no time order of activity is preserved?