The time taken for a transistor to turn from saturation to cut off is directly proportional to charge carriers.

Explanation:
A transistor is a three-layer semiconductor device that acts as an electronic switch. It consists of three regions: the emitter, base, and collector. The transistor operates in three different regions: active, saturation, and cut off.

In the saturation region, the transistor is fully turned on, and there is a large current flowing through it. In the cut off region, the transistor is fully turned off, and there is no current flowing through it. The transition from saturation to cut off or vice versa occurs when the base-emitter junction is forward or reverse biased, respectively.

The time taken for this transition is determined by the charge carriers present in the base region of the transistor. The base region acts as a control region, and the movement of charge carriers (electrons or holes) in this region determines the state of the transistor.

When the transistor is in saturation, a large number of charge carriers are injected into the base region, which keeps the transistor conducting. However, when the transistor needs to turn off and enter the cut off state, these charge carriers need to be removed from the base region.

The time taken for the charge carriers to recombine or diffuse out of the base region depends on their concentration and mobility. If there are more charge carriers present in the base region, it will take more time for them to leave, resulting in a longer transition time. On the other hand, if there are fewer charge carriers, the transition time will be shorter.

Therefore, the time taken for a transistor to turn from saturation to cut off is directly proportional to the number of charge carriers present in the base region. This means that if there are more charge carriers, the transition time will be longer, and if there are fewer charge carriers, the transition time will be shorter.

Hence, the correct answer is option 'B' - directly proportional to charge carriers.