**Core Concept**
The crown completion of deciduous teeth is a critical aspect of dental development, which involves the formation of the tooth structure. The development of deciduous teeth is influenced by the timing and sequence of various morphological events, including the formation of the crown and root.

**Why the Correct Answer is Right**
Crown completion of deciduous mandibular second molars occurs between 11 to 13 years of age. This is a significant milestone in dental development, marking the end of the crown formation process. The timing of crown completion is influenced by the eruption of the second molars, which typically occurs between 12 to 16 years of age. The development of the mandibular second molars is also influenced by the presence of the third molars, which can cause crowding and other orthodontic issues if they erupt too late.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a specific age range for crown completion of deciduous mandibular second molars.

**Option B:** This option is incorrect because it does not accurately describe the typical age range for crown completion of deciduous mandibular second molars.

**Option C:** This option is incorrect because it is not a valid age range for crown completion of deciduous mandibular second molars.

**Clinical Pearl / High-Yield Fact**
It is essential for clinicians to be aware of the typical age range for crown completion of deciduous teeth to accurately diagnose and treat dental development anomalies. Failure to recognize these anomalies can lead to complications such as malocclusion and orthodontic issues.

**Correct Answer: D.**