Which of the following is observed in the $C_2H_4$ molecule?

Arrange the following according to the instructions:
$(i)$ Arrange in descending order of acidic strength:
$CH_3COOH, (CH_3)_3CCOOH, (CH_3)_2CHCOOH, CH_3CH_2COOH$
(ii) Arrange in descending order of stability:
$\stackrel{+}{CH}_3, (CH_3)_3\stackrel{+}{C}, CH_3CH_2^+, (CH_3)_2\stackrel{+}{CH}$
(iii) Arrange in increasing order of acidic strength:
$CCl_3COOH, CH_3COOH, CHCl_2COOH, CH_2ClCOOH$
(iv) Arrange in increasing order of stability:
$(a)$ $CH_3CH=CH-CHO$
$(b)$ $CH_3\stackrel{+}{CH}-CH=C-O:^-$
$(c)$ $\stackrel{+}{CH}_2-CH=CH-C=O$
$(v)$ Arrange in decreasing order of stability:
$(I)$ $CH_2=CH-CHO$
$(II)$ $\stackrel{+}{CH}_2-CH=C-H$ (with $O^-$)
$(III)$ $:CH_2-CH=C-H$ (with $O^-$)

The structure of triphenylmethyl cation is given below. This is very stable and some of its salts can be stored for months. Explain the cause of high stability of this cation.

Select the correct set of substituents for the existence of $-M$ and negative inductive $(-I)$ effects.

Arrange the following carbanions in decreasing order of stability $:$

What is the correct order of acidity of the protons marked $H_A, H_B, H_C,$ and $H_D$ in the given compound?