The way to figure this out is first to know what ionic means: metal + non-metal. This eliminates A, since metals are columns 1-2A and the transition metals.
Now you need to see how far apart each of the other atoms are from each other on the periodic table.
O-O is neutral since each O has same electronegativity value.
N-O is right next to each other and on same row (period) while C-O are two spaces away from each other (more polar).
Si-O is also ƒᴀʀтher away, but since Si is in the next period, its electronegativity is much lower (valence electrons are further away from nucleus).
So C-O is your answer, and it is based on the idea that electronegativity inceases as one moves up and to the right on the periodic table (with F the most electronegative).
Hope this helps for you to apply on an exam!
[B] C-O is the answer
A is ionic. C is non polar. D is weak polar and E is not as polar as B.