Final answer:
The correct fluid for a rear differential is most likely to be 75w. This grade indicates a high-viscosity gear oil suitable for the high load and lubrication requirements within a differential.
Step-by-step explanation:
The fluid most likely to go in a rear differential is D) 75w. The 'w' in the grade of the fluid stands for 'winter' and reflects the viscosity, or thickness, of the oil which is important for the gears in the differential. Differential fluid, often referred to as gear oil, is thicker so it can properly lubricate the moving parts within the differential. Unlike engine oil which typically has lower viscosity grades like 0w, 5w, or 10w, differentials require a higher viscosity fluid like 75w to endure the heavy loads and protect against wear and tear.