**Core Concept**
The question is testing the relative radiosensitivity of different tissues in the human body. Radiosensitivity is the susceptibility of cells or tissues to damage from ionizing radiation, with some tissues being more sensitive to radiation than others. This concept is crucial in radiation therapy and radiation protection.

**Why the Correct Answer is Right**
The least radio-sensitive tissue is bone tissue. Bone tissue has a low rate of cell division, which makes it less susceptible to radiation-induced damage. Additionally, bone cells, such as osteocytes, are relatively resistant to radiation due to their ability to repair DNA damage. This is in contrast to rapidly dividing cells, such as those found in the gastrointestinal tract and bone marrow, which are highly sensitive to radiation.

**Why Each Wrong Option is Incorrect**
**Option A:**
Bone marrow is actually one of the most radiosensitive tissues, as it contains rapidly dividing cells that are highly susceptible to radiation-induced damage.

**Option B:**
The skin is moderately radiosensitive, but not the least sensitive. It contains a mix of rapidly and slowly dividing cells, making it more sensitive to radiation than bone tissue.

**Clinical Pearl / High-Yield Fact**
When treating cancer with radiation, it's essential to minimize exposure to radiosensitive tissues to avoid unnecessary damage and side effects. This is why radiation oncologists carefully plan treatment to target the tumor while sparing surrounding tissues.

**Correct Answer: A. Bone marrow**