**Core Concept**
Thumb-sucking is a common habit in children, especially during the pre-school stage, often related to anxiety, stress, or self-soothing mechanisms. This habit can lead to dental and orthodontic problems if not addressed.

**Why the Correct Answer is Right**
In the pre-school stage, approximately 25-30% of children exhibit thumb-sucking behavior. This is due to the child's attempt to self-soothe and cope with stress or anxiety. The habit is more prevalent in children who have a family history of thumb-sucking or who have experienced separation anxiety.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is too low, as thumb-sucking is a relatively common habit in pre-school children.
**Option B:** This option is not a specific percentage and does not provide a clear answer.
**Option C:** This option is too high, as while a significant percentage of children exhibit thumb-sucking, 50% is an overestimation.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize that thumb-sucking is a normal developmental phase, but prolonged thumb-sucking can lead to dental problems such as overbite and malocclusion.

**Correct Answer:** A. 25-30%