**Core Concept**
The question is testing the understanding of the radiation tolerance of different tissues, specifically the buccal mucosa. Radiation tolerance is the ability of tissues to withstand radiation without causing damage. The buccal mucosa is more sensitive to radiation than skin due to its rapid cellular turnover.

**Why the Correct Answer is Right**
The buccal mucosa is a rapidly proliferating tissue, with a high rate of cellular turnover. This means that there is a constant supply of new cells being produced to replace the old ones. As a result, the buccal mucosa is more sensitive to radiation than other tissues like skin, which has a slower rate of cellular turnover. When exposed to radiation, the rapidly dividing cells of the buccal mucosa are more likely to be damaged, leading to a radiation reaction. This is because radiation damages DNA and interferes with cell division, causing cells to die or become dysfunctional.

**Why Each Wrong Option is Incorrect**
**Option A:** Rapid cellular turn-over in bone is not directly related to the radiation sensitivity of the buccal mucosa. While bone is a dynamic tissue with a high rate of cellular turnover, the question is specifically asking about the buccal mucosa.

**Option B:** Slow cellular turn-over in blood vessel is actually the opposite of what is true for the buccal mucosa. Blood vessels have a slower rate of cellular turnover compared to the buccal mucosa, which is not relevant to the question.

**Option C:** Rapid cellular turn-over in skin is not accurate. While skin does have a certain rate of cellular turnover, it is slower compared to the buccal mucosa. This is why the buccal mucosa exhibits radiation reactions before the skin.

**Clinical Pearl / High-Yield Fact**
Remember that rapidly proliferating tissues like the buccal mucosa are more sensitive to radiation due to their high rate of cellular turnover. This is a key concept in radiation oncology, as it helps clinicians predict which tissues are more likely to be damaged by radiation.

**✓ Correct Answer: D. Rapid cellular turn-over in mucosa**