In the $IUPAC$ nomenclature system,what is the correct decreasing order of priority for the following functional groups?

What is the chemical formula of the ethyl group represented as $CH_3CH_2$?

Select the correct $IUPAC$ name of pyrogallol.

Select the correct match of $IUPAC$ name with structure.

Give the $IUPAC$ names of the following compounds:
$(i)$ $Ph-CH_2-CH_2-COOH$
$(ii)$ $(CH_3)_2C=CH-COOH$
$(iii)$ Structure of $2-$Methylcyclopentanecarboxylic acid
$(iv)$ Structure of $2,4,6-$Trinitrobenzoic acid

Given below are two statements:
Statement $I$: $IUPAC$ name of $HO-CH_2-(CH_2)_3-CH_2-COCH_3$ is $7-$hydroxyheptan$-2-$one.
Statement $II$: $2-$oxoheptan$-7-$ol is the correct $IUPAC$ name for above compound.
In the light of the above statements,choose the most appropriate answer from the options given below: