which of the following substances is least able to cross the blood-brain barrier.) a. waterb. sodium ionsc glucosed. white blood cells

d. white blood cells

Explanation:

It is the white blood cells that have the least tendency to cross the blood - brain barrier.

Blood - brain barrier is psychological barrier and restricts substances that circulate into the blood from crossing over the central nervous system.

Water, glucose as well as sodium ions can easily cross the barrier. As these are good for body supply energy to the brain.