Which type of the overlap of orbitals is involved in hyperconjugation?

With respect to the compounds $I-V$,choose the correct statement$(s)$.
$(A)$ The acidity of compound $I$ is due to delocalization in the conjugate base.
$(B)$ The conjugate base of compound $IV$ is aromatic.
$(C)$ Compound $II$ becomes more acidic,when it has a $-NO_2$ substituent.
$(D)$ The acidity of compounds follows the order $IV > V > I > II > III$.

Rank the following compounds in the order of increasing acidic strength:

Formic acid is a stronger acid than acetic acid. This can be explained using

Given below are two statements $:$
Statement $I :$ Hyperconjugation is not a permanent effect.
Statement $II :$ In general,greater the number of alkyl groups attached to a positively charged $C$ atom,greater is the hyperconjugation interaction and stabilization of the cation.
In the light of the above statements,choose the correct answer from the options given below

The decreasing order of acidity of the following terminal acetylenes is: