In IUPAC nomenclature, the correct order of decreasing priority of...

The correct order of decreasing priority for functional groups in IUPAC nomenclature is:

Carboxylic acids ($-COOH$) $>$ Sulphonic acids ($-SO_3H$) $>$ Esters ($-COOR$) $>$ Acid halides ($-COX$) $>$ Amides ($-CONH_2$) $>$ Nitriles ($-CN$) $>$ Aldehydes ($-CHO$) $>$ Ketones ($>C=O$) $>$ Alcohols ($-OH$) $>$ Thiols ($-SH$) $>$ Amines ($-NH_2$) $>$ Alkynes ($-C\equiv C-$).

Checking the given options:
Option (A) is incorrect because $-CHO$ has a higher priority than $>C=O$.
Option (B) is incorrect because $-COOCH_3$ has a higher priority than $-CONH_2$.
Option (C) correctly follows the priority order: $-CONH_2 > -CHO > >C=O > -NH_2 > -C\equiv C-$.
Option (D) is incorrect because $-CN$ has a higher priority than $-CHO$.

Answer: $-CONH_2, -CHO, >C=O, -NH_2, -C\equiv C-$