The correct answer is B. becomes.
The air pressure decreases as you go higher in the atmosphere. This means that there are fewer air molecules per unit volume, and so there is less oxygen available for you to breathe. This is why it is more difficult to breathe at high altitudes.
The verb “becomes” is the most accurate way to describe this change. The air pressure does not suddenly change from one value to another; it gradually decreases as you go higher. This means that the difficulty of breathing also gradually increases.
The other options are not as accurate. The verb “has become” suggests that the difficulty of breathing is a permanent change. However, the difficulty of breathing is only temporary; it goes away when you return to a lower altitude. The verb “is becoming” suggests that the difficulty of breathing is a gradual change. However, the difficulty of breathing actually changes quite quickly as you go higher in the atmosphere. The verb “became” suggests that the difficulty of breathing is a past event. However, the difficulty of breathing is an ongoing event; it happens every time you go higher in the atmosphere.