**Core Concept**
Weber's test is a clinical examination used to assess lateralization of sound in patients with hearing loss. It involves placing a vibrating tuning fork on the midline of the patient's skull, and the patient is asked to indicate where the sound is heard. The test relies on the principle of sound conduction through bone and air, and the patient's ability to lateralize the sound to the affected ear.

**Why the Correct Answer is Right**
For Weber's test to lateralize to the affected ear, there must be a significant loss of hearing in that ear, typically greater than 20-30 dB. This loss of hearing causes the sound to be perceived as louder in the affected ear, resulting in lateralization. The mechanism behind this is the increased sound conduction through bone, which bypasses the damaged middle ear structures.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because a loss of hearing of less than 20 dB may not cause significant lateralization on Weber's test.
* **Option B:** This option is incorrect because a conductive hearing loss may not cause lateralization on Weber's test, as sound conduction through bone is not affected.
* **Option C:** This option is incorrect because a sensorineural hearing loss may cause a positive Rinne test, but not necessarily lateralization on Weber's test.

**Clinical Pearl / High-Yield Fact**
A significant loss of hearing (>20-30 dB) is required for Weber's test to lateralize to the affected ear. This is a critical point to remember when interpreting the results of Weber's test in patients with hearing loss.

**Correct Answer: C. 20-30 dB.**