Standard Methods for the Examination of Dairy Products (SMEDP) and BIS specify that thermophilic bacterial spore and vegetative counts in dairy products (such as milk powder, evaporated milk, and pasteurized milk) are enumerated by plating on Standard Methods Agar and incubating at $55 \pm 1^\circ\text{C}$ for $48 \pm 3\text{ hours}$.

Option analysis:

• $42^\circ\text{C}$ for $24\text{ hours}$: Parameter used for thermotolerant coliforms or yoghurt culture counts.
• $42^\circ\text{C}$ for $48\text{ hours}$: Not the standard thermophilic incubation protocol.
• $55^\circ\text{C}$ for $24\text{ hours}$: Insufficient time for full colony formation of slow-growing thermophiles.
• $55^\circ\text{C}$ for $48\text{ hours}$: Correct. Standard incubation condition for thermophilic count in dairy bacteriology.