**Core Concept**
Thumb sucking and bruxism in a child can be symptoms of a deeper neurodevelopmental issue. This habit can be associated with anxiety, stress, or other underlying psychological factors that affect the child's oral motor function.

**Why the Correct Answer is Right**
The probable cause of thumb sucking and bruxism in this child is a manifestation of an anxiety disorder, specifically **Separation Anxiety Disorder (SAD)**. Children with SAD often exhibit self-soothing behaviors like thumb sucking or bruxism as a coping mechanism for their anxiety. This habit can also be a sign of an underlying attachment disorder or an insecure attachment style.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because thumb sucking and bruxism are not typically associated with **Attention Deficit Hyperactivity Disorder (ADHD)**. While children with ADHD may exhibit oral motor dysfunctions, these specific habits are not characteristic of the disorder.

**Option B:** This option is incorrect because **Enuresis** (bedwetting) is not directly related to thumb sucking and bruxism. While both conditions can be symptoms of underlying psychological issues, they are distinct and separate problems.

**Option C:** This option is incorrect because **Sleep Apnea** is a sleep-related breathing disorder that affects respiratory function, not oral motor function or habits. Thumb sucking and bruxism are not typically associated with sleep apnea.

**Option D:** This option is incorrect because **Gastroesophageal Reflux Disease (GERD)** is a gastrointestinal condition that affects the esophagus and stomach, not the oral motor function or habits of a child.

**Clinical Pearl / High-Yield Fact**
It's essential to note that thumb sucking and bruxism can be symptoms of an underlying anxiety disorder, and a thorough evaluation by a pediatrician or child psychologist is necessary to determine the root cause of these habits.

**Correct Answer:** D.