Which of these bonds is most polar? C-H, N-H, O-H, C-O?

The O-H bond is the most polar bond.

Explanation:

Because there is the biggest different in electronegativity.

c. because step up transformer steps up the input voltage i.e increases the input voltage  

the number of turns are greater on the secondary side of step up transformer  

transformer does not handle dc current

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)