**Core Concept**
The World Health Organization (WHO) and other health authorities have established guidelines for optimal lighting levels in various settings to prevent eye strain and promote overall visual health. These guidelines consider factors such as task type, age, and individual sensitivity.

**Why the Correct Answer is Right**
The recommended illumination for general office work is indeed 400 lux. This level of lighting is sufficient for most tasks, including reading, writing, and computer work, without causing eye strain or discomfort. The 400 lux threshold is also in line with the WHO's guidelines for visual comfort and productivity. In contrast, higher levels of illumination may be necessary for tasks that require more detailed work, such as surgery or fine craftsmanship.

**Why Each Wrong Option is Incorrect**
* **Option A:** 100 lux is too low for general office work, as it may cause eye strain and fatigue, particularly for tasks that require prolonged visual attention.
* **Option C:** 900 lux is excessively high and may cause discomfort, headaches, or even eye damage in individuals who are sensitive to bright light.
* **Option D:** 1000 lux is also too high for general office work, as it may lead to glare and visual discomfort, especially in environments with reflective surfaces.

**Clinical Pearl / High-Yield Fact**
When designing or optimizing lighting in an office or workspace, consider the 80:20 rule: 80% of the time, the lighting should be sufficient for 20% of the tasks. This principle helps ensure that the lighting is balanced and suitable for a variety of activities.

**✓ Correct Answer: B. 400 lux**