FIR input frequencies
The input frequencies are a free choice by the question setter, in order to explore what the filter is doing. 

It would have been equally as valid for the question to say the input frequencies to the system are w_k = {0, 1, 4}. Obviously with different input frequencies, the filter response would be different. Hint, often specific input frequencies are chosen that hit 'interesting' features of filters, or where particular factors cancel out nicely.