**Core Concept**
The process of acid etching is crucial in creating a strong bond between the tooth surface and the pit and fissure sealant. This is achieved by demineralizing the enamel and creating micro-retentions, allowing for better adhesion and durability of the sealant.

**Why the Correct Answer is Right**
The American Dental Association (ADA) recommends acid etching for 15 seconds to create optimal micro-retentions for the sealant. This duration allows for sufficient demineralization of the enamel without causing excessive damage to the tooth structure. The etching process involves the use of phosphoric acid, which reacts with the calcium ions in the enamel, creating a surface that is conducive to the adhesion of the sealant.

**Why Each Wrong Option is Incorrect**
**Option A:** 30 seconds - This duration is excessive and may lead to over-demineralization of the enamel, compromising the sealant's adhesion and the tooth's integrity.
**Option B:** 10 seconds - This duration is inadequate and may not provide sufficient micro-retentions for the sealant to adhere properly.
**Option C:** 20 seconds - This duration is close but still not the recommended time, and may not provide the optimal micro-retentions for the sealant.

**Clinical Pearl / High-Yield Fact**
Remember the "15-second rule" for acid etching before placing a pit and fissure sealant, as this duration provides the optimal balance between demineralization and tooth integrity.

**Correct Answer: C. 20 seconds**