In direct microscopic examination (DMC) of milk films, morphological types reflect specific contamination sources:

• Large rod-shaped bacteria and spore-forming bacilli (such as Bacillus species) originate predominantly from environmental dust, dirt, bedding material, soil, and feed.
• Lactic streptococci in pairs/short chains indicate poor cooling.
• Massive clumped cocci or streptococci associated with leucocytes indicate mastitis.
• Bacterial clumps on utensils reflect improper cleaning.
  
  Option analysis:
• Improperly cleaned utensils: Typically introduces masses of mixed cocci and non-sporeforming bacteria embedded in milkstone.
• Dusty and dirty environment: Correct. Soil and dust are the principal sources of spore-forming rod-shaped bacilli in raw milk.
• Improper cooling of milk: Characterized by rapid proliferation of lactic streptococci.
• Mastitis: Characterized by excessive somatic cell/leucocyte counts and long-chain streptococci or staphylococci.

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.