## Core Concept
The question pertains to the standards for workplace illumination, specifically focusing on the recommended range for regular work. Illumination levels are measured in foot-candles, which quantify the amount of light cast on a surface.

## Why the Correct Answer is Right
The correct illumination range for regular work, such as reading, writing, and using computers, generally falls between 30 to 50 foot-candles. This range supports tasks that require moderate visual acuity without causing eye strain. Levels that are too low can lead to visual fatigue, while levels that are too high can cause glare and discomfort.

## Why Each Wrong Option is Incorrect
- **Option A:** 10-20 foot-candles is typically considered adequate for tasks that require low visual acuity, such as storage areas or general movement, but not sufficient for regular work that involves detailed tasks.
- **Option B:** 15-25 foot-candles might be on the lower end for regular work but could be suitable for tasks requiring low to moderate visual acuity. However, it does not cover the optimal range for most regular work activities.
- **Option D:** 100-150 foot-candles would be excessively high for regular work and more suitable for tasks requiring high visual acuity, such as surgery or detailed assembly work.

## Clinical Pearl / High-Yield Fact
A key point to remember is that proper illumination is crucial for reducing eye strain and improving productivity. The American National Standards Institute (ANSI) and the Illuminating Engineering Society of North America (IESNA) provide guidelines for recommended illumination levels for various tasks and environments.

## Correct Answer: C. 30-50.