IUPAC name of picric acid is :
a.) 2,4,6 - Trinitrobenzene carboxylic acid
b.) 2,4 - Dinitrobenzene carboxylic acid
c.) 2,4,6 - Trinitrophenol
d.) 2,4 - Dinitrophenol

Hint: The picric acid consists of one OH group which has the first priority. So, the carbon with hydroxyl group will be numbered one. There are three nitro groups present at ortho and para position to the OH group.

Complete answer:
We know that picric acid is one of the strong acids known.
We have the structure of picric acid as -

If we see IUPAC nomenclature, then OH group is senior in priority list. So, it should be given the first number. Then, there are three nitro groups present.
On numbering of the ring, we get the IUPAC name of picric acid as-
2,4,6 - Trinitrophenol

Thus, the option c.) is the correct answer.

Additional information:
The picric acid is one of strong acidic phenols. It is a pale yellow solid that is odourless. It has various properties. It can be used as a yellow dye or as an antiseptic
It has much use in military explosives. It is used in Bouin solution which is used as a fixative for histology study. Picric acid was earlier used to treat burns, malaria, herpes and smallpox etc.

Note:
The IUPAC stands for International Union of Pure and Applied Chemistry. It has formulated some rules for the systematic nomenclature of organic compounds which govern the naming of all the organic compounds. This makes IUPAC names of all compounds the same.