What does this phrase mean: "last resort"?

[Elar]

Test No. incompl/inter-94 "Letter of Complaint", question 1

I am writing this letter to you as a sort of last ......... since all my attempts to phone you

(a) result
(b) return
(c) report
(d) resort

Test No. incompl/inter-94 "Letter of Complaint", answer 1

I am writing this letter to you as a sort of last resort since all my attempts to phone you

Correct answer: (d) resort

Your answer was: incorrect
I am writing this letter to you as a sort of last result since all my attempts to phone you
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Hi, Please, Why not "result"?
thanks

[Torsten]

Elar,

If you want to achieve a certain goal and you use all kinds of methods but you fail every time, you may try something as a last restort. So, that's a fixed expression.
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