• Regulatory Standard for Aflatoxin $M_1$: The Food Safety and Standards Authority of India (FSSAI) Food Safety and Standards (Contaminants, Toxins and Residues) Regulations mandate a maximum permissible limit of $0.5\ \mu\text{g/kg}$ ($0.5\text{ ppb}$) for Aflatoxin $M_1$ in milk and milk products.
• Metabolic Origin: Aflatoxin $M_1$ is the hydroxylated 4-hydroxy metabolite of Aflatoxin $B_1$ ($AFB_1$). When dairy cattle ingest feed contaminated with Aspergillus flavus or Aspergillus parasiticus, approximately $1\text{--}3\%$ of dietary $AFB_1$ is bio-transformed in the liver by cytochrome P450 enzymes and excreted into milk as $AFM_1$.
• Public Health Hazard: Aflatoxin $M_1$ is a Group 2B / Group 1 hepatocarcinogen that is completely heat-resistant, surviving standard pasteurization, boiling, and spray drying.

Correct option: (2) 0.5 ppb