My opinion is that the candidate should give the answer with a reason that he is interested in this particular area and wants to serve the nation through this job. He may also relate some personal contact/experience or motivation towards the job particulars.
For example, if I have to answer, I would say:
"Although I've done M.Sc Economics, but to join Police Department esp. at the senior/elite level has been my desire. I did M. Sc as I am interested in getting good education. I think better education widens the horizons of one's approach and thoughts towards any matter, and this is much needed in every deptt. as much as in the Police. I believe I can better serve having the professional degree instead having simple/low level of educational degree. (My uncle/father etc is in Police deptt. and this also inspires me to serve in this deptt.)
Just ad-lib thoughts of mine.

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"But screw your courage to the sticking place,
And we'll not fail." _Shakespeare, 'Macbeth')