**Core Concept**
The limit of loudness is a critical factor in preventing noise-induced hearing loss (NIHL), a common occupational hazard affecting workers exposed to high-intensity sounds. The decibel scale measures the loudness of sounds, with higher values indicating greater intensity. Prolonged exposure to sounds above a certain threshold can cause permanent damage to the hair cells in the cochlea.

**Why the Correct Answer is Right**
The correct answer is D. 85 decibels. According to the National Institute for Occupational Safety and Health (NIOSH), workers should be limited to an 8-hour time-weighted average exposure of 85 decibels to prevent NIHL. This is based on the understanding that sounds above 85 decibels can cause permanent damage to the hair cells in the cochlea, leading to hearing loss. The 85-decibel limit is also in line with the Occupational Safety and Health Administration (OSHA) regulations in the United States.

**Why Each Wrong Option is Incorrect**
**Option A:** 55 decibels is actually considered a relatively quiet level of sound, equivalent to a normal conversation between two people. Prolonged exposure to sounds at this level is unlikely to cause NIHL.

**Option B:** 65 decibels is still within the lower range of sound levels and is not considered hazardous for workers. For example, a vacuum cleaner or a lawnmower typically produces sounds in the range of 60-70 decibels.

**Option C:** 75 decibels is closer to the threshold for potential NIHL, but it is still below the recommended limit. For example, a food blender or a chain saw can produce sounds in the range of 70-80 decibels.

**Clinical Pearl / High-Yield Fact**
A useful mnemonic to remember the 85-decibel limit is "The 8-Hour 85 Rule": workers should be limited to an 8-hour exposure to sounds at or below 85 decibels to prevent NIHL.

**Correct Answer Line**
✓ Correct Answer: D. 85.