Question

Add formal charges to each resonance form of hcno below.based on the formal charges you added above, which structure is favored?

Answer 1

Q1)
formal charges for atoms are assigned, this charge can be calculated using the following formula;
f.c = number of valence electrons - non bonding electrons around the atom - number of bonds

3 structures have been given, lets calculate for each structure 
A. 
charges for the atoms 
 H = 1-0-1=0
C = 4 -4-2 = -2
N = 5 - 0-4 = 1
O = 6 - 2 - 3 = 1

B.
H = 1-0-1 = 0
C = 4 - 0 - 4 =0
N = 5 - 0 - 4 = 1
O = 6 - 6 - 1 = -1

C.
H = 1-0-1 = 0
C = 4 - 2 - 3 = -1
N = 5 - 0 - 4 = 1
O = 6 - 4 - 2 = 0

Q2)
next we need to see which structure is most favoured.
the structure with least number of atoms that have a charge.
structure B and C have 2 atoms with charges whereas A has three atoms with charges, including an atom with -2 charge which is not common. so structure A can be disregarded.
another rule is that the more electronegative atom should have the more negative charge than less electronegative atoms. In structure C, Carbon the less electronegative atom compared to N and O, has -1 value and its more negative than the charges on N and O.
structure B on the other hand, O the more electronegative atom has -1 charge and C has 0 charge. Therefore the more favoured structure is B.