What meteorological condition is quantified by "visibility less than 1/4 mile"?

The condition quantified by "visibility less than 1/4 mile" is dense fog. This is because dense fog significantly reduces visibility, making it difficult to see more than a short distance. Fog forms when moisture in the air condenses into tiny water droplets, creating an obscuring effect in the atmosphere.

In contrast, while heavy rain can also reduce visibility, it typically does not result in visibility as severely low as that caused by dense fog unless there is extreme rain. Snow flurries can create some visibility issues, but they usually do not lead to visibility dropping below 1/4 mile unless accompanied by additional conditions such as strong winds or other significant weather phenomena. Severe thunderstorms can cause low visibility due to heavy rain, hail, or debris but again, this would require extreme conditions to match the low visibility defined in the question.

Therefore, the defining characteristic of dense fog is its ability to create extremely low visibility levels, often below 1/4 mile, clearly making it the correct answer for this question.