Biuret test is used to test the presence of : 

The correct answer is - 3) Protein

Concept:

Fat -

• Fats are a type of lipid consisting of triesters of glycerol and fatty acids or triglycerides. 

Carbohydrates -

• Carbohydrates are defined as “optically active polyhydroxy aldehydes or ketones or the compounds which produce units of such type on hydrolysis”. 

Protein - 

• A protein is a naturally occurring, extremely complex substance that consists of amino acid residues joined by peptide bonds.

Nucleic Acid - 

• Nucleic acids are polynucleotides that are, long chainlike molecules composed of a series of nearly identical building blocks called nucleotides.
• Each nucleotide consists of a nitrogen-containing aromatic base attached to a pentose (five-carbon) sugar, which is in turn attached to a phosphate group

Explanation:

Biuret test - The biuret test is a chemical test that can be used to check for the presence of peptide bonds in a given analyte.

• Therefore, the biuret test can also be used to gauge the amount of protein present in the analyte.
• In this test, the presence of peptides results in the formation of pale purple-coloured coordination compounds of the copper (II) ion (when the solution is sufficiently alkaline). 
• The copper (II) present in the reaction binds itself to the nitrogen atoms that are present in the protein peptides. 

Therefore, the Biuret test is used to test the presence of Protein.