What are the characteristics of resonance structures?

Identify the relationship between the following two structures: $\begin{matrix} \overset{\Theta}{\overset{\centerdot \,\centerdot}{C}}H_2-C(=O)-CH_3 \end{matrix}$ and $\begin{matrix} CH_2=C(O^{\Theta})CH_3 \end{matrix}$

Which of the following statements are true for the nitrate ion $(NO_3^-)$?
$(a)$ The coordination number of $N$ is $3$,and the structure is trigonal planar.
$(b)$ The formal charge on $N$ is $+1$.
$(c)$ The average formal charge on each $O$ atom is $-2/3$.
$(d)$ The average bond order of the $NO$ bond is $4/3$.
$(e)$ All $NO$ bond lengths are identical due to resonance.
Select the correct code:

Which statement is true regarding the $PO_4^{3-}$ ion?
$A$. In $PO_4^{3-}$ ion,the $P-O$ bond order is $1.25$.
$B$. In $PO_4^{3-}$,the formal charge on each $O$ atom is $-0.75$.
$C$. $PO_4^{3-}$ shows four resonating structures.

The increasing order of stability of the resonance structures is:

Compare the relative stability of the following resonating structures:
$(i)$ $CH_3-CH=C(CN)-COCH_3$
(ii) $CH_3-CH=C(CN)-C^+(O^-)CH_3$
(iii) $CH_3-C^+(H)-C(CN)=C(O^-)CH_3$
(Note: The structures are provided in the image.)