Which of the following statements are $TRUE$?
$(I)$ Inductive effect operates through the $\sigma$ bond
$(II)$ Resonance effect operates through the $\pi$ bond
$(III)$ Inductive effects operate through the $\pi$ bond
$(IV)$ Resonance effect operates through the $\sigma$ bond

Orbital interaction between the $\sigma$ bonds of a substituent group and a neighbouring $\pi$ orbital is known as

Given below are two statements:
Statement $I$: In $O_2N-C_6H_4-CH^+-C_6H_4-OCH_3$, the carbocation is stabilised by $+R$ effect of $-OCH_3$ group.
Statement $II$: In $O_2N-C_6H_4-CH^--C_6H_4-OCH_3$, the carbanion is stabilised by $-R$ effect of $-NO_2$ group.
In the light of the above statements, choose the correct answer from the options given below:

Which one of the nitrogen atoms in $H_2N-NH-CO-NH_2$ (where the atoms are labeled $I, II, III$ as shown in the structure) is the most nucleophilic?

Fill in the blanks:
$(1)$ In $+E$ effect,$\pi$ electrons from a multiple bond migrate to the ........... .
$(2)$ In $-E$ effect,$\pi$ electrons from a multiple bond migrate to the ........... .
$(3)$ ........... and ........... increase stability.
$(4)$ ........... inductive effect increases the acidic strength and ........... inductive effect decreases the strength of $-COOH$.

Rank the hydrogen atoms $(H_a, H_b, H_c)$ present in the following molecule in decreasing order of their acidic strength.