6. a is the transition zone between air masses of different densities

a mid-latitude cyclone
b front
c polar low
d high pressure system

Option (B)

Explanation:

The front often refers to the transition zone between the air masses that are comprised of different densities. It extends both vertically as well as horizontally. The passing of a front over a particular area depicts the change in its weather pattern. These fronts are responsible for the various weather events that cause hurricanes, tornadoes, and heavy rainfall.

There are basically 4 types of fronts, namely-

(1) Cold front

(2) Warm front

(3) Stationary front

(4) Occluded front

These front types occur when the air mass of variable densities moves from one place to another.

Thus, the correct answer is option (B).