**Core Concept**
Fluoride fortification in salt is a public health measure aimed at preventing dental caries by increasing fluoride intake through dietary sources. This concept is part of community water fluoridation and salt fluoridation programs worldwide.

**Why the Correct Answer is Right**
Fluoride is added to salt in the form of sodium fluoride or potassium fluoride at a concentration of 250-400 ppm (parts per million). This is done to reduce the incidence of dental caries by increasing the fluoride ion concentration in the oral cavity. The fluoride ions help in making tooth enamel more resistant to acid attacks from plaque bacteria and sugars in the mouth. The recommended concentration of fluoride in salt is between 250-400 ppm.

**Why Each Wrong Option is Incorrect**
**Option A:** 150 ppm is too low to have a significant effect on dental caries prevention.
**Option B:** 600 ppm is too high and may cause dental fluorosis, a condition characterized by mottling of the teeth.
**Option C:** No fluoride is added to salt in some countries, but this does not make it the standard concentration.

**Clinical Pearl / High-Yield Fact**
The optimal concentration of fluoride in salt is between 250-400 ppm to prevent dental caries without causing dental fluorosis.

**Correct Answer: D. 250-400 ppm**