Sulphonamides act as

Sulphonamides are synthetic drugs that contain the $$-SO_2NH-$$ group attached to an aromatic ring. They resemble para-aminobenzoic acid (PABA), a compound that many bacteria need to synthesise folic acid, which in turn is essential for the production of cofactors required for DNA replication.

Because sulphonamides are structural analogues of PABA, they competitively inhibit the enzyme that incorporates PABA into folic-acid biosynthesis. When the pathway is blocked, the bacteria cannot multiply and eventually die or are unable to cause infection. Mammalian (human) cells are not affected because they obtain folic acid pre-formed from the diet instead of synthesising it, so the drug shows selective toxicity towards microorganisms.

Hence, sulphonamides prevent the growth and proliferation of pathogenic bacteria; in pharmacology such agents are termed antimicrobials (or more precisely, antibacterial chemotherapeutic agents).

Therefore, among the given options, sulphonamides act as antimicrobials.

Option C which is: Antimicrobials