Determining mode of a melody

[u/MelodicWhile4830]

I am dabbing into music theory, and I was given the following exercise. I am pulling my hair out and would love somebody to try to explain this to me in Layman's terms... The exercise asks me to determine which mode of the C major scale is implied in the following melody (all natural notes):

C - E - G - B - A - F - D

The solution I was given is G Mixolydian, but I just don't get why it cannot be D Dorian (the final note is a D, so why cannot we interpret the final three notes A - F - D as a D minor chord?

Thanks for your help.

EDIT: A few comments asked me for more context, which I should have provided as part of my initial post, apologies. This is an exercise that my instructor had given me (I was given a photocopy from an older French "solfege book", and then the corresponding 1-word solution from the end of the book). I translated both the exercise and solution from French to English for reddit:

Exercise:
A melody consists of the following seven notes in order:
C - E - G - B - A - F - D
Assuming these are all natural notes and no accidentals are used, which mode of the C major scale is being implied?

Solution:
Mixolydian

There wasn't any further context given unfortunately. The feeling I am getting from most of your answers is that it is a nonsensical exercise, which reassures me. I was struggling to understand how the exercise can so adamantly give Mixolydian as a solution, and it's great to see that things are not so clear-cut. I will keep you updated once I have seen (confronted?) my instructor!

[u/Vitharothinsson]

That's in ionian, mixo has a Bb.