The correct decreasing order of priority of functional groups in naming an organic compound as per IUPAC system of nomenclature is

We need to find the correct decreasing order of priority of functional groups in IUPAC nomenclature according to IUPAC nomenclature rules, when a molecule has multiple functional groups, the principal characteristic group is chosen based on a defined priority order. The standard IUPAC priority order (from highest to lowest) for some common functional groups is:

$$-COOH > -SO_3H > -COCl > -COOR > -CONH_2 > -CN > -CHO > -C(=O)- > -OH > -NH_2$$

Next, we examine each option.

Option A: $$-COOH > -CONH_2 > -COCl > -CHO$$. Since this places $$-CONH_2$$ (amide) above $$-COCl$$ (acid chloride), it is incorrect because acid halides ($$-COCl$$) have higher priority than amides ($$-CONH_2$$) in the IUPAC system. The correct relative order among these is: $$-COOH > -COCl > -CONH_2 > -CHO$$.

Option B: $$-SO_3H > -COCl > -CONH_2 > -CN$$. We verify each pair against the IUPAC priority table:

- $$-SO_3H$$ (sulphonic acid) has very high priority, ranked just below $$-COOH$$. It is correctly placed first here.

- $$-COCl$$ (acid chloride) is next in the table after sulphonic acid derivatives. Correctly placed second.

- $$-CONH_2$$ (amide) comes after acid halides and esters. Correctly placed third.

- $$-CN$$ (nitrile/cyanide) comes after amides. Correctly placed fourth.

Every adjacent pair follows the standard IUPAC priority. This sequence is correct.

Option C: $$-COOH > -COOR > -COCl > -CONH_2$$. This places $$-COOR$$ (ester) above $$-COCl$$ (acid chloride) and is incorrect because acid chlorides rank higher than esters in the IUPAC priority order.

Option D: $$-COOH > -COOR > -CONH_2 > -COCl$$. This places $$-COCl$$ at the lowest priority among these groups and is incorrect because acid chlorides have higher priority than both esters and amides.

Therefore, the correct answer is Option B: $$-SO_3H > -COCl > -CONH_2 > -CN$$.