The turn-off time of a transistor is the sum of the storage time and the fall time.

Explanation:
Transistors are electronic devices that are widely used in various electronic circuits. They can be used as amplifiers, switches, and oscillators. The turn-off time of a transistor refers to the time it takes for the transistor to switch from the on-state to the off-state. This time is important as it affects the switching speed and performance of the transistor.

The turn-off time can be divided into two components - the storage time and the fall time.

1. Storage Time:
The storage time of a transistor is the time it takes for the excess charge carriers to be removed from the base region when the transistor is turning off. During the on-state of the transistor, the base-emitter junction is forward biased, allowing current to flow. When the transistor is turned off, this bias is removed, and the excess charge carriers in the base region need to be removed before the transistor can fully turn off. The storage time is the time it takes for this process to occur.

2. Fall Time:
The fall time of a transistor is the time it takes for the output voltage to transition from a high voltage level to a low voltage level when the transistor is turning off. This transition occurs due to the discharge of the load capacitance connected to the transistor's output. The fall time is influenced by factors such as the load capacitance, the impedance of the load, and the characteristics of the transistor.

Therefore, the turn-off time of a transistor is the sum of the storage time and the fall time. It is important to consider both of these components when designing circuits that involve switching transistors to ensure proper switching behavior and minimize any delays or distortions in the output signals.

In conclusion, the correct answer is option 'A' - the turn-off time of a transistor is the sum of the storage time and the fall time. These two components contribute to the overall turn-off time and should be taken into account when analyzing and designing transistor circuits.