Final answer:
Increased blood flow resistance is caused by increased blood pressure.
Step-by-step explanation:
In the case of blood flow resistance, increased blood vessel length and increased blood pressure contribute to higher resistance and lower flow. On the other hand, decreased blood viscosity and vasodilation both lead to lower resistance and increased flow.
For example, if the blood vessels become longer, the surface area of the vessel increases, impeding the flow of blood and resulting in higher resistance. Similarly, if blood pressure increases, it will exert more force against the vessel walls, also increasing resistance. In contrast, if blood viscosity decreases or the blood vessels dilate, the resistance decreases, allowing for increased blood flow.
Therefore, the correct answer is D. increased blood pressure as it contributes to increased blood flow resistance.