• WHO Drinking Water Standards for Lead: Under the World Health Organization (WHO) Guidelines for Drinking-water Quality, the provisional guideline maximum permissible concentration for Lead ($Pb$) is $10\ \mu\text{g/L}$ ($0.01\text{ mg/L}$).
• Toxicological Rationale: Lead is a potent cumulative neurotoxic heavy metal causing cognitive deficits, encephalopathy, peripheral neuropathy, and nephropathy, especially in infants and young children, whose intestinal absorption is substantially higher than in adults.

Correct option: (1) 10 pg/L