**Core Concept**
The question is testing the relationship between the diameter of a blood vessel and its resistance to blood flow, according to Poiseuille's law. This law states that the resistance to blood flow in a vessel is inversely proportional to the fourth power of its radius.

**Why the Correct Answer is Right**
When the diameter of a vessel is doubled, its radius is also doubled. According to Poiseuille's law, resistance (R) is inversely proportional to the fourth power of the radius (r): R ∝ 1/r^4. If the radius is doubled, the new resistance (R') is R' ∝ 1/(2r)^4 = 1/16R. This means that the resistance to blood flow decreases by a factor of 16 when the diameter is doubled. This is because the increased diameter reduces the frictional resistance to blood flow.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because doubling the diameter of a vessel does not increase its resistance. In fact, the resistance decreases.
* **Option B:** This option is incorrect because it suggests that the resistance increases when the diameter is doubled, which is the opposite of the correct answer.
* **Option C:** This option is incorrect because it suggests that the resistance remains the same when the diameter is doubled, which is also incorrect.

**Clinical Pearl / High-Yield Fact**
Remember that the resistance to blood flow in a vessel is inversely proportional to the fourth power of its radius. This means that small changes in the radius of a vessel can have a significant impact on its resistance to blood flow.

**Correct Answer: D. The resistance to blood flow decreases by a factor of 16.**