• Incubation Standards for Thermophilic Counts in Milk:
• Standard Microbiological Protocol (APHA / ISO / BIS IS: 1479):
• Thermophilic bacteria and flat-sour sporeformers (such as Geobacillus stearothermophilus) grow optimally above $50^\circ\text{C}$.
• For enumeration of thermophilic bacterial counts in milk, plates are officially incubated at $55 \pm 1^\circ\text{C}$ for $48\text{ hours}$.
• In contrast, mesophilic total viable counts are incubated at $30 - 37^\circ\text{C}$, and psychrotrophs at $7^\circ\text{C}$.
• Conclusion: 55°C (Option 4).

Correct option: (4) 55°C